pathspec: stop calling ensure_full_index

The add_pathspec_matches_against_index() focuses on matching a pathspec
to file entries in the index. This already works correctly for its only
use: checking if untracked files exist in the index.

The compatibility checks in t1092 already test that 'git add <dir>'
works for a directory outside of the sparse cone. That provides coverage
for removing this guard.

This finalizes our ability to run 'git add .' without expanding a sparse
index to a full one. This is evidenced by an update to t1092 and by
these performance numbers for p2000-sparse-operations.sh:

Test                                    HEAD~1            HEAD
--------------------------------------------------------------------------------
2000.10: git add . (full-index-v3)      0.37(0.28+0.07)   0.36(0.27+0.06) -2.7%
2000.11: git add . (full-index-v4)      0.33(0.26+0.06)   0.32(0.28+0.05) -3.0%
2000.12: git add . (sparse-index-v3)    0.57(0.53+0.07)   0.06(0.06+0.07) -89.5%
2000.13: git add . (sparse-index-v4)    0.57(0.53+0.07)   0.05(0.03+0.09) -91.2%

While the ~90% improvement is shown by the test results, it is worth
noting that expanding the sparse index was adding overhead in previous
commits. Comparing to the full index case, we see the performance go
from 0.33s to 0.05s, an 85% improvement.

Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

pathspec.c

@@ -37,8 +37,6 @@ void add_pathspec_matches_against_index(const struct pathspec *pathspec,
 			num_unmatched++;
 	if (!num_unmatched)
 		return;
-	/* TODO: audit for interaction with sparse-index. */
-	ensure_full_index(istate);
 	for (i = 0; i < istate->cache_nr; i++) {
 		const struct cache_entry *ce = istate->cache[i];
 		if (sw_action == PS_IGNORE_SKIP_WORKTREE && ce_skip_worktree(ce))

t/t1092-sparse-checkout-compatibility.sh

@@ -322,9 +322,6 @@ test_expect_success 'commit including unstaged changes' '
 test_expect_success 'status/add: outside sparse cone' '
 	init_repos &&
 
-	# adding a "missing" file outside the cone should fail
-	test_sparse_match test_must_fail git add folder1/a &&
-
 	# folder1 is at HEAD, but outside the sparse cone
 	run_on_sparse mkdir folder1 &&
 	cp initial-repo/folder1/a sparse-checkout/folder1/a &&
@@ -652,7 +649,9 @@ test_expect_success 'sparse-index is not expanded' '
 	echo >>sparse-index/README.md &&
 	ensure_not_expanded add -A &&
 	echo >>sparse-index/extra.txt &&
-	ensure_not_expanded add extra.txt
+	ensure_not_expanded add extra.txt &&
+	echo >>sparse-index/untracked.txt &&
+	ensure_not_expanded add .
 '
 
 # NEEDSWORK: a sparse-checkout behaves differently from a full checkout