Merge pull request #425: Sparse index: add more performance tests

One thing I forgot when talking about the sparse index is that we have a performance test: `t/perf/p2000-sparse-operations.sh`. This test wasn't helpful for commands like `git merge` that need a particular set of input, but work for more read-only operations.

Here is a quick demonstration of how this performance test works so we could have a definitive measure of how your previous updates improved performance. 

To get these results, I ran the following command in `t/perf`:

```
 ./run 4bcd533 f9255a5 f28fc01 b713582 -- p2000-sparse-operations.sh
```

The short-shas correspond to the merge commits for these PRs:

* #410 
* #421 
* #417 
* #419

The test takes a copy of the Git repository and creates several copies within a nested directory heirarchy.


```
Test                                                   4bcd533       f9255a5              f28fc01              b713582           
-------------------------------------------------------------------------------------------------------------------------------------------------
2000.2: git status (full-v3)                           0.19(0.15+0.05)   0.19(0.16+0.05) +0.0%    0.20(0.18+0.03) +5.3%    0.19(0.17+0.04) +0.0% 
2000.3: git status (full-v4)                           0.20(0.18+0.04)   0.19(0.15+0.06) -5.0%    0.21(0.18+0.05) +5.0%    0.18(0.18+0.02) -10.0%
2000.4: git status (sparse-v3)                         0.04(0.04+0.04)   0.05(0.07+0.04) +25.0%   0.04(0.04+0.05) +0.0%    0.04(0.06+0.04) +0.0% 
2000.5: git status (sparse-v4)                         0.04(0.03+0.06)   0.04(0.05+0.05) +0.0%    0.05(0.05+0.04) +25.0%   0.05(0.06+0.04) +25.0%
2000.6: git add -A (full-v3)                           0.36(0.29+0.05)   0.38(0.28+0.07) +5.6%    0.36(0.31+0.05) +0.0%    0.37(0.31+0.05) +2.8% 
2000.7: git add -A (full-v4)                           0.34(0.27+0.06)   0.34(0.29+0.05) +0.0%    0.34(0.29+0.04) +0.0%    0.35(0.28+0.06) +2.9% 
2000.8: git add -A (sparse-v3)                         0.06(0.07+0.04)   0.06(0.05+0.06) +0.0%    0.06(0.09+0.01) +0.0%    0.06(0.08+0.03) +0.0% 
2000.9: git add -A (sparse-v4)                         0.05(0.05+0.04)   0.05(0.05+0.07) +0.0%    0.05(0.04+0.06) +0.0%    0.06(0.06+0.05) +20.0%
2000.10: git add . (full-v3)                           0.38(0.31+0.05)   0.37(0.29+0.06) -2.6%    0.37(0.30+0.07) -2.6%    0.37(0.29+0.06) -2.6% 
2000.11: git add . (full-v4)                           0.35(0.31+0.04)   0.35(0.29+0.07) +0.0%    0.35(0.29+0.05) +0.0%    0.34(0.29+0.06) -2.9% 
2000.12: git add . (sparse-v3)                         0.06(0.06+0.05)   0.06(0.05+0.06) +0.0%    0.06(0.07+0.05) +0.0%    0.06(0.09+0.03) +0.0% 
2000.13: git add . (sparse-v4)                         0.06(0.06+0.06)   0.06(0.07+0.04) +0.0%    0.05(0.06+0.05) -16.7%   0.05(0.05+0.07) -16.7%
2000.14: git commit -a -m A (full-v3)                  0.48(0.37+0.08)   0.45(0.36+0.08) -6.2%    0.45(0.35+0.09) -6.2%    0.44(0.36+0.07) -8.3% 
2000.15: git commit -a -m A (full-v4)                  0.45(0.40+0.06)   0.43(0.34+0.07) -4.4%    0.45(0.37+0.06) +0.0%    0.42(0.36+0.05) -6.7% 
2000.16: git commit -a -m A (sparse-v3)                0.05(0.05+0.06)   0.05(0.05+0.03) +0.0%    0.05(0.06+0.06) +0.0%    0.05(0.04+0.06) +0.0% 
2000.17: git commit -a -m A (sparse-v4)                0.05(0.06+0.03)   0.05(0.06+0.04) +0.0%    0.06(0.07+0.05) +20.0%   0.05(0.04+0.06) +0.0% 
2000.18: git checkout -f - (full-v3)                   0.55(0.43+0.08)   0.54(0.46+0.05) -1.8%    0.55(0.46+0.07) +0.0%    0.54(0.40+0.10) -1.8% 
2000.19: git checkout -f - (full-v4)                   0.55(0.41+0.09)   0.50(0.40+0.09) -9.1%    0.51(0.46+0.05) -7.3%    0.51(0.44+0.06) -7.3% 
2000.20: git checkout -f - (sparse-v3)                 0.06(0.09+0.03)   0.06(0.08+0.03) +0.0%    0.06(0.06+0.05) +0.0%    0.07(0.09+0.03) +16.7%
2000.21: git checkout -f - (sparse-v4)                 0.06(0.08+0.04)   0.05(0.07+0.05) -16.7%   0.05(0.07+0.04) -16.7%   0.06(0.09+0.03) +0.0% 
```

All of the above were already integrated.

```
2000.22: git reset (full-v3)                           0.41(0.32+0.06)   0.40(0.31+0.06) -2.4%    0.41(0.33+0.05) +0.0%    0.42(0.34+0.04) +2.4% 
2000.23: git reset (full-v4)                           0.37(0.32+0.05)   0.35(0.30+0.05) -5.4%    0.37(0.30+0.05) +0.0%    0.35(0.31+0.03) -5.4% 
2000.24: git reset (sparse-v3)                         0.68(0.65+0.05)   0.55(0.52+0.04) -19.1%   0.04(0.05+0.04) -94.1%   0.04(0.05+0.04) -94.1%
2000.25: git reset (sparse-v4)                         0.70(0.65+0.05)   0.54(0.50+0.06) -22.9%   0.04(0.07+0.01) -94.3%   0.03(0.05+0.05) -95.7%
2000.26: git reset --hard (full-v3)                    0.54(0.43+0.07)   0.53(0.43+0.06) -1.9%    0.55(0.46+0.05) +1.9%    0.55(0.44+0.06) +1.9% 
2000.27: git reset --hard (full-v4)                    0.50(0.45+0.03)   0.50(0.43+0.05) +0.0%    0.49(0.41+0.06) -2.0%    0.50(0.42+0.05) +0.0% 
2000.28: git reset --hard (sparse-v3)                  0.83(0.76+0.06)   0.68(0.62+0.05) -18.1%   0.07(0.05+0.02) -91.6%   0.07(0.05+0.02) -91.6%
2000.29: git reset --hard (sparse-v4)                  0.80(0.75+0.05)   0.69(0.62+0.06) -13.8%   0.07(0.04+0.02) -91.2%   0.07(0.04+0.03) -91.2%
```

As expected, `git reset [--hard]` improves with the sparse index integration, but remains constant across the full index case.

```
2000.30: git update-index --add --remove (full-v3)     0.03(0.01+0.01)   0.03(0.02+0.01) +0.0%    0.03(0.02+0.01) +0.0%    0.03(0.01+0.01) +0.0% 
2000.31: git update-index --add --remove (full-v4)     0.03(0.02+0.01)   0.03(0.02+0.01) +0.0%    0.03(0.03+0.00) +0.0%    0.03(0.02+0.01) +0.0% 
2000.32: git update-index --add --remove (sparse-v3)   0.57(0.54+0.02)   0.43(0.42+0.00) -24.6%   0.44(0.41+0.03) -22.8%   0.44(0.42+0.01) -22.8%
2000.33: git update-index --add --remove (sparse-v4)   0.56(0.52+0.04)   0.43(0.42+0.01) -23.2%   0.44(0.42+0.02) -21.4%   0.42(0.41+0.01) -25.0%
```

These do not change significantly because #423 is not merged.

```
2000.34: git diff (full-v3)                            0.07(0.05+0.03)   0.06(0.05+0.03) -14.3%   0.07(0.05+0.03) +0.0%    0.06(0.05+0.03) -14.3%
2000.35: git diff (full-v4)                            0.06(0.05+0.03)   0.06(0.05+0.02) +0.0%    0.06(0.05+0.02) +0.0%    0.06(0.06+0.02) +0.0% 
2000.36: git diff (sparse-v3)                          0.25(0.23+0.03)   0.17(0.17+0.02) -32.0%   0.18(0.18+0.02) -28.0%   0.01(0.03+0.03) -96.0%
2000.37: git diff (sparse-v4)                          0.25(0.22+0.05)   0.16(0.16+0.01) -36.0%   0.18(0.15+0.04) -28.0%   0.01(0.04+0.02) -96.0%
2000.38: git diff --staged (full-v3)                   0.03(0.01+0.01)   0.03(0.02+0.01) +0.0%    0.03(0.02+0.01) +0.0%    0.03(0.02+0.00) +0.0% 
2000.39: git diff --staged (full-v4)                   0.04(0.03+0.01)   0.03(0.02+0.01) -25.0%   0.03(0.03+0.00) -25.0%   0.03(0.03+0.00) -25.0%
2000.40: git diff --staged (sparse-v3)                 0.21(0.19+0.01)   0.15(0.13+0.01) -28.6%   0.15(0.14+0.01) -28.6%   0.01(0.01+0.00) -95.2%
2000.41: git diff --staged (sparse-v4)                 0.22(0.21+0.01)   0.14(0.11+0.03) -36.4%   0.15(0.13+0.02) -31.8%   0.01(0.01+0.00) -95.5%
```

The `git diff` improvements are measurable.

```
2000.42: git sparse-checkout reapply (full-v3)         0.63(0.54+0.05)   0.56(0.48+0.04) -11.1%   0.57(0.48+0.03) -9.5%    0.59(0.48+0.05) -6.3% 
2000.43: git sparse-checkout reapply (full-v4)         0.60(0.54+0.02)   0.51(0.46+0.03) -15.0%   0.54(0.48+0.02) -10.0%   0.50(0.44+0.04) -16.7%
2000.44: git sparse-checkout reapply (sparse-v3)       0.91(0.86+0.05)   0.05(0.05+0.00) -94.5%   0.06(0.05+0.01) -93.4%   0.06(0.06+0.00) -93.4%
2000.45: git sparse-checkout reapply (sparse-v4)       0.92(0.88+0.04)   0.05(0.05+0.00) -94.6%   0.05(0.05+0.01) -94.6%   0.05(0.04+0.01) -94.6%
```

Finally, the `git sparse-checkout` measurements are also present.

This test script is particularly valuable when contributing changes upstream. It can be good to start by adding the lines to the performance test in an early commit, then demonstrating the performance change by copying the necessary lines from the output table into your commit message.

t/perf/p2000-sparse-operations.sh

@@ -55,7 +55,7 @@ test_expect_success 'setup repo and indexes' '
 	git -c core.sparseCheckoutCone=true clone --branch=wide --sparse . full-v3 &&
 	(
 		cd full-v3 &&
-		git sparse-checkout init --cone &&
+		git sparse-checkout init --cone --no-sparse-index &&
 		git sparse-checkout set $SPARSE_CONE &&
 		git config index.version 3 &&
 		git update-index --index-version=3 &&
@@ -64,7 +64,7 @@ test_expect_success 'setup repo and indexes' '
 	git -c core.sparseCheckoutCone=true clone --branch=wide --sparse . full-v4 &&
 	(
 		cd full-v4 &&
-		git sparse-checkout init --cone &&
+		git sparse-checkout init --cone --no-sparse-index &&
 		git sparse-checkout set $SPARSE_CONE &&
 		git config index.version 4 &&
 		git update-index --index-version=4 &&
@@ -123,5 +123,6 @@ test_perf_on_all git blame $SPARSE_CONE/f3/a
 test_perf_on_all git read-tree -mu HEAD
 test_perf_on_all git checkout-index -f --all
 test_perf_on_all git update-index --add --remove $SPARSE_CONE/a
+test_perf_on_all git sparse-checkout reapply
 
 test_done