checkout: stop expanding sparse indexes

Previous changes did the necessary improvements to unpack-trees.c and
diff-lib.c in order to modify a sparse index based on its comparision
with a tree. The only remaining work is to remove some
ensure_full_index() calls and add tests that verify that the index is
not expanded in our interesting cases. Include 'switch' and 'restore' in
these tests, as they share a base implementation with 'checkout'.

Here are the relevant performance results from
p2000-sparse-operations.sh:

Test                                     HEAD~1           HEAD
--------------------------------------------------------------------------------
2000.18: git checkout -f - (full-v3)     0.49(0.43+0.03)  0.47(0.39+0.05) -4.1%
2000.19: git checkout -f - (full-v4)     0.45(0.37+0.06)  0.42(0.37+0.05) -6.7%
2000.20: git checkout -f - (sparse-v3)   0.76(0.71+0.07)  0.04(0.03+0.04) -94.7%
2000.21: git checkout -f - (sparse-v4)   0.75(0.72+0.04)  0.05(0.06+0.04) -93.3%

It is important to compare the full index case to the sparse index case,
as the previous results for the sparse index were inflated by the index
expansion. For index v4, this is an 88% improvement.

On an internal repository with over two million paths at HEAD and a
sparse-checkout definition containing ~60,000 of those paths, 'git
checkout' went from 3.5s to 297ms with this change. The theoretical
optimum where only those ~60,000 paths exist was 275ms, so the extra
sparse directory entries contribute a 22ms overhead.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

builtin/checkout.c

@@ -378,9 +378,6 @@ static int checkout_worktree(const struct checkout_opts *opts,
 	if (pc_workers > 1)
 		init_parallel_checkout();
 
-	/* TODO: audit for interaction with sparse-index. */
-	ensure_full_index(&the_index);
-
 	for (pos = 0; pos < active_nr; pos++) {
 		struct cache_entry *ce = active_cache[pos];
 		if (ce->ce_flags & CE_MATCHED) {
@@ -530,8 +527,6 @@ static int checkout_paths(const struct checkout_opts *opts,
 	 * Make sure all pathspecs participated in locating the paths
 	 * to be checked out.
 	 */
-	/* TODO: audit for interaction with sparse-index. */
-	ensure_full_index(&the_index);
 	for (pos = 0; pos < active_nr; pos++)
 		if (opts->overlay_mode)
 			mark_ce_for_checkout_overlay(active_cache[pos],
@@ -1593,6 +1588,9 @@ static int checkout_main(int argc, const char **argv, const char *prefix,
 
 	git_config(git_checkout_config, opts);
 
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
+
 	opts->track = BRANCH_TRACK_UNSPECIFIED;
 
 	if (!opts->accept_pathspec && !opts->accept_ref)

t/t1092-sparse-checkout-compatibility.sh

@@ -560,7 +560,15 @@ test_expect_success 'sparse-index is not expanded' '
 	echo >>sparse-index/a &&
 	ensure_not_expanded commit --include a -m a &&
 	echo >>sparse-index/deep/deeper1/a &&
-	ensure_not_expanded commit --include deep/deeper1/a -m deeper
+	ensure_not_expanded commit --include deep/deeper1/a -m deeper &&
+	ensure_not_expanded checkout rename-out-to-out &&
+	ensure_not_expanded checkout - &&
+	ensure_not_expanded switch rename-out-to-out &&
+	ensure_not_expanded switch - &&
+	git -C sparse-index reset --hard &&
+	ensure_not_expanded checkout rename-out-to-out -- deep/deeper1 &&
+	git -C sparse-index reset --hard &&
+	ensure_not_expanded restore -s rename-out-to-out -- deep/deeper1
 '
 
 # NEEDSWORK: a sparse-checkout behaves differently from a full checkout