blame: enable and test the sparse index

Enable the sparse index for the 'git blame' command. The index was already
not expanded with this command, so the most interesting thing to do is to
add tests that verify that 'git blame' behaves correctly when the sparse
index is enabled and that its performance improves. More specifically, these
cases are:

1. The index is not expanded for 'blame' when given paths in the sparse
checkout cone at multiple levels.

2. Performance measurably improves for 'blame' with sparse index when given
paths in the sparse checkout cone at multiple levels.

The `p2000` tests demonstrate an ~60% execution time reduction when running
'blame' for a file two levels deep and and a ~30% execution time reduction
for a file three levels deep.

Test                                         before  after
----------------------------------------------------------------
2000.62: git blame f2/f4/a (full-v3)         0.31    0.32 +3.2%
2000.63: git blame f2/f4/a (full-v4)         0.29    0.31 +6.9%
2000.64: git blame f2/f4/a (sparse-v3)       0.55    0.23 -58.2%
2000.65: git blame f2/f4/a (sparse-v4)       0.57    0.23 -59.6%
2000.66: git blame f2/f4/f3/a (full-v3)      0.77    0.85 +10.4%
2000.67: git blame f2/f4/f3/a (full-v4)      0.78    0.81 +3.8%
2000.68: git blame f2/f4/f3/a (sparse-v3)    1.07    0.72 -32.7%
2000.99: git blame f2/f4/f3/a (sparse-v4)    1.05    0.73 -30.5%

We do not include paths outside the sparse checkout cone because blame
currently does not support blaming files outside of the sparse definition.
Attempting to do so fails with the following error:

fatal: no such path '<path outside sparse definition>' in HEAD

Signed-off-by: Lessley Dennington <lessleydennington@gmail.com>

builtin/blame.c

@@ -902,6 +902,9 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 	long anchor;
 	const int hexsz = the_hash_algo->hexsz;
 
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
+
 	setup_default_color_by_age();
 	git_config(git_blame_config, &output_option);
 	repo_init_revisions(the_repository, &revs, NULL);

t/perf/p2000-sparse-operations.sh

@@ -115,5 +115,7 @@ test_perf_on_all git reset --hard
 test_perf_on_all git reset -- does-not-exist
 test_perf_on_all git diff
 test_perf_on_all git diff --staged
+test_perf_on_all git blame $SPARSE_CONE/a
+test_perf_on_all git blame $SPARSE_CONE/f3/a
 
 test_done

t/t1092-sparse-checkout-compatibility.sh

@@ -485,15 +485,16 @@ test_expect_success 'blame with pathspec inside sparse definition' '
 	test_all_match git blame deep/deeper1/deepest/a
 '
 
-# TODO: blame currently does not support blaming files outside of the
-# sparse definition. It complains that the file doesn't exist locally.
-test_expect_failure 'blame with pathspec outside sparse definition' '
+# Blame does not support blaming files outside of the sparse
+# definition, so we verify this scenario.
+test_expect_success 'blame with pathspec outside sparse definition' '
 	init_repos &&
 
-	test_all_match git blame folder1/a &&
-	test_all_match git blame folder2/a &&
-	test_all_match git blame deep/deeper2/a &&
-	test_all_match git blame deep/deeper2/deepest/a
+	test_sparse_match git sparse-checkout set &&
+	test_sparse_match test_must_fail git blame folder1/a &&
+	test_sparse_match test_must_fail git blame folder2/a &&
+	test_sparse_match test_must_fail git blame deep/deeper2/a &&
+	test_sparse_match test_must_fail git blame deep/deeper2/deepest/a
 '
 
 test_expect_success 'checkout and reset (mixed)' '
@@ -871,6 +872,15 @@ test_expect_success 'sparse-index is not expanded: merge conflict in cone' '
 	)
 '
 
+test_expect_success 'sparse index is not expanded: blame' '
+	init_repos &&
+
+	ensure_not_expanded blame a &&
+	ensure_not_expanded blame deep/a &&
+	ensure_not_expanded blame deep/deeper1/a &&
+	ensure_not_expanded blame deep/deeper1/deepest/a
+'
+
 # NEEDSWORK: a sparse-checkout behaves differently from a full checkout
 # in this scenario, but it shouldn't.
 test_expect_success 'reset mixed and checkout orphan' '