update-index: remove unnecessary index expansion in do_reupdate

Instead of ensuring the full index, only need to prevent reupdating sparse
directory entries to maintain expected behavior. Corresponding addition to
index expansion test verifies the sparse index is not expanded.

Signed-off-by: Victoria Dye <vdye@github.com>

builtin/update-index.c

@@ -747,17 +747,23 @@ static int do_reupdate(int ac, const char **av,
 		 * commit.  Update everything in the index.
 		 */
 		has_head = 0;
+
  redo:
-	/* TODO: audit for interaction with sparse-index. */
-	ensure_full_index(&the_index);
 	for (pos = 0; pos < active_nr; pos++) {
 		const struct cache_entry *ce = active_cache[pos];
 		struct cache_entry *old = NULL;
 		int save_nr;
 		char *path;
 
-		if (ce_stage(ce) || !ce_path_match(&the_index, ce, &pathspec, NULL))
+		/*
+		 * We can safely skip re-updating sparse directories because if there
+		 * were any changes to re-update inside of the sparse directory, it
+		 * would not be sparse.
+		 */
+		if (S_ISSPARSEDIR(ce->ce_mode) || ce_stage(ce) ||
+		    !ce_path_match(&the_index, ce, &pathspec, NULL))
 			continue;
+
 		if (has_head)
 			old = read_one_ent(NULL, &head_oid,
 					   ce->name, ce_namelen(ce), 0);

t/t1092-sparse-checkout-compatibility.sh

@@ -1392,6 +1392,9 @@ test_expect_success 'sparse index is not expanded: update-index' '
 	ensure_not_expanded update-index --add README.md &&
 	ensure_not_expanded update-index a &&
 	ensure_not_expanded update-index --remove deep/a &&
+
+	rm -f sparse-index/README.md sparse-index/a &&
+	ensure_not_expanded update-index --add --remove --again
 '
 
 # NEEDSWORK: a sparse-checkout behaves differently from a full checkout