stash: avoid sparse-index expansion for in-cone paths

`git stash push -- <pathspec>` expands a sparse index before checking
whether the pathspec matches any tracked paths. This is unnecessary
when the pathspec is wholly inside the sparse-checkout cone and makes
a path-limited stash proportional to the size of the full index.

Use `pathspec_needs_expanded_index()` to expand only when a pathspec
can match part of a sparse-directory entry, as `git rm` and `git
reset` already do. Keep the full-index behavior for pathspecs that
need it.

Add compatibility coverage for literal, prefixed, wildcard, file,
multiple, staged, and missing pathspecs. Add the corresponding
path-limited stash case to p2000.

On a cone-mode repository with 349,525 tracked paths and 49 sparse
index entries, the best of three runs changed from 18.87s to 0.06s.
Trace2 reported four index expansions before this change and none
after it.

Signed-off-by: Ted Nyman <tnyman@openai.com>
Reviewed-by: Taylor Blau <ttaylorr@openai.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: tnm-oai

builtin/stash.c

@@ -1702,8 +1702,8 @@ static int do_push_stash(const struct pathspec *ps, const char *stash_msg, int q
 	if (!include_untracked && ps->nr) {
 		char *ps_matched = xcalloc(ps->nr, 1);
 
-		/* TODO: audit for interaction with sparse-index. */
-		ensure_full_index(the_repository->index);
+		if (pathspec_needs_expanded_index(the_repository->index, ps))
+			ensure_full_index(the_repository->index);
 		for (size_t i = 0; i < the_repository->index->cache_nr; i++)
 			ce_path_match(the_repository->index, the_repository->index->cache[i], ps,
 				      ps_matched);

t/perf/p2000-sparse-operations.sh

@@ -108,6 +108,7 @@ test_perf_on_all () {
 
 test_perf_on_all git status
 test_perf_on_all 'git stash && git stash pop'
+test_perf_on_all "git stash push -- $SPARSE_CONE/a && git stash pop"
 test_perf_on_all 'echo >>new && git stash -u && git stash pop'
 test_perf_on_all git add -A
 test_perf_on_all git add .

t/t1092-sparse-checkout-compatibility.sh

@@ -1598,6 +1598,61 @@ test_expect_success 'sparse-index is not expanded: stash' '
 	ensure_not_expanded stash pop
 '
 
+test_expect_success 'sparse-index is not expanded: stash in-cone pathspec' '
+	init_repos &&
+
+	echo unrelated >>sparse-index/deep/e &&
+	echo literal >>sparse-index/deep/a &&
+	ensure_not_expanded stash push -- deep/a &&
+	test_grep ! literal sparse-index/deep/a &&
+	test_grep unrelated sparse-index/deep/e &&
+	ensure_not_expanded stash pop &&
+	test_grep literal sparse-index/deep/a &&
+
+	echo prefixed >>sparse-index/deep/a &&
+	ensure_not_expanded -C deep stash push -- a &&
+	test_grep ! prefixed sparse-index/deep/a &&
+	test_grep unrelated sparse-index/deep/e &&
+	ensure_not_expanded stash pop &&
+	test_grep prefixed sparse-index/deep/a &&
+
+	echo wildcard >>sparse-index/deep/a &&
+	ensure_not_expanded stash push -- "deep/a*" &&
+	test_grep ! wildcard sparse-index/deep/a &&
+	test_grep unrelated sparse-index/deep/e &&
+	ensure_not_expanded stash pop &&
+	test_grep wildcard sparse-index/deep/a &&
+
+	echo pathspec-file >>sparse-index/deep/a &&
+	echo deep/a >pathspec-file &&
+	ensure_not_expanded stash push --pathspec-from-file=../pathspec-file &&
+	test_grep ! pathspec-file sparse-index/deep/a &&
+	test_grep unrelated sparse-index/deep/e &&
+	ensure_not_expanded stash pop &&
+	test_grep pathspec-file sparse-index/deep/a &&
+
+	echo multiple-a >>sparse-index/deep/a &&
+	echo multiple-e >>sparse-index/deep/e &&
+	ensure_not_expanded stash push -- deep/a deep/e &&
+	test_grep ! multiple-a sparse-index/deep/a &&
+	test_grep ! multiple-e sparse-index/deep/e &&
+	ensure_not_expanded stash pop &&
+	test_grep multiple-a sparse-index/deep/a &&
+	test_grep multiple-e sparse-index/deep/e &&
+
+	echo staged >>sparse-index/deep/a &&
+	git -C sparse-index add deep/a &&
+	ensure_not_expanded stash push --staged -- deep/a &&
+	test_grep ! staged sparse-index/deep/a &&
+	test_grep unrelated sparse-index/deep/e &&
+	ensure_not_expanded stash pop --index &&
+	test_grep staged sparse-index/deep/a &&
+	test_must_fail git -C sparse-index diff --cached --quiet -- deep/a &&
+
+	ensure_not_expanded ! stash push -- deep/does-not-exist &&
+	test_grep "did not match any file" sparse-index-error
+'
+
 test_expect_success 'describe tested on all' '
 	init_repos &&