sparse-index: mark am, mv, submodule, grep as sparse-index compatible

These commands were found to have never declared command_requires_full_index.
After analysis, all four are safe with sparse index (= 0).

git-am: The patch application path uses index_name_pos() which auto-expands
sparse directory entries on demand when a specific file path is looked up.
The three-way merge fallback uses merge-ort, which works from tree OIDs and
handles sparse directories natively.  refresh_index() explicitly skips
S_ISSPARSEDIR entries.  git-rebase --apply invokes git-am as a child
process; rebase itself already declares command_requires_full_index = 0.

git-mv: For in-cone moves, all index lookups use index_name_pos() on
paths that cannot be ancestors of sparse directory entries (by
definition of the sparse cone), so no expansion is triggered.
index_range_of_same_dir() iterates only in-cone entries in this case.
For out-of-cone moves (--sparse flag), a targeted ensure_full_index()
is needed because the bulk cache[] iteration and rename_index_entry_at()
assume individual file entries, not collapsed directory stubs.

git-submodule--helper: Submodule entries (S_ISGITLINK) are never collapsed
into sparse directory entries -- convert_to_sparse_rec() in sparse-index.c
explicitly blocks directories containing gitlinks from being collapsed.
module_list_compute() filters on S_ISGITLINK, skipping any sparse
directory entries.  module_add() already calls ensure_full_index()
unconditionally before its index iteration.

grep (submodule path): grep_cache() explicitly handles S_ISSPARSEDIR
entries by reading the tree object and recursing via grep_tree(), so
sparse directory contents are grepped correctly.  The subrepo index
is independent of the superproject.

Author: spkrka

builtin/am.c

@@ -2465,9 +2465,8 @@ int cmd_am(int argc,
 	/* Ensure a valid committer ident can be constructed */
 	git_committer_info(IDENT_STRICT);
 
-	/* not yet verified whether this can use the sparse index */
 	prepare_repo_settings(the_repository);
-	the_repository->settings.command_requires_full_index = 1;
+	the_repository->settings.command_requires_full_index = 0;
 
 	if (repo_read_index_preload(the_repository, NULL, 0) < 0)
 		die(_("failed to read the index"));

builtin/grep.c

@@ -500,9 +500,8 @@ static int grep_submodule(struct grep_opt *opt,
 	 *
 	 * Note that this list is not exhaustive.
 	 */
-	/* not yet verified whether subrepo can use the sparse index */
 	prepare_repo_settings(subrepo);
-	subrepo->settings.command_requires_full_index = 1;
+	subrepo->settings.command_requires_full_index = 0;
 
 	repo_read_gitmodules(subrepo, 0);
 

builtin/mv.c

@@ -28,6 +28,7 @@
 #include "strvec.h"
 #include "submodule.h"
 #include "entry.h"
+#include "sparse-index.h"
 
 static const char * const builtin_mv_usage[] = {
 	N_("git mv [-v] [-f] [-n] [-k] <source> <destination>"),
@@ -248,14 +249,16 @@ int cmd_mv(int argc,
 	if (--argc < 1)
 		usage_with_options(builtin_mv_usage, builtin_mv_options);
 
-	/* not yet verified whether this can use the sparse index */
 	prepare_repo_settings(the_repository);
-	the_repository->settings.command_requires_full_index = 1;
+	the_repository->settings.command_requires_full_index = 0;
 
 	repo_hold_locked_index(the_repository, &lock_file, LOCK_DIE_ON_ERROR);
 	if (repo_read_index(the_repository) < 0)
 		die(_("index file corrupt"));
 
+	if (ignore_sparse)
+		ensure_full_index(the_repository->index);
+
 	internal_prefix_pathspec(&sources, prefix, argv, argc, 0);
 	CALLOC_ARRAY(modes, argc);
 

builtin/submodule--helper.c

@@ -3833,9 +3833,8 @@ int cmd_submodule__helper(int argc,
 	};
 	argc = parse_options(argc, argv, prefix, options, usage, 0);
 
-	/* not yet verified whether this can use the sparse index */
 	prepare_repo_settings(the_repository);
-	the_repository->settings.command_requires_full_index = 1;
+	the_repository->settings.command_requires_full_index = 0;
 
 	return fn(argc, argv, prefix, repo);
 }