sparse-index: declare command_requires_full_index for undeclared commands

These four code paths were found to never declare their
command_requires_full_index value.  Set them to 1 (require full index
expansion) as a safe default, with comments noting they have not yet
been verified for sparse-index compatibility.

Found by changing the default to -1 (unset) and adding an accessor
that dies when a command reads the index without first declaring its
sparse-index requirement.

Author: spkrka

builtin/am.c

@@ -23,6 +23,7 @@
 #include "lockfile.h"
 #include "cache-tree.h"
 #include "refs.h"
+#include "repo-settings.h"
 #include "commit.h"
 #include "diff.h"
 #include "unpack-trees.h"
@@ -2464,6 +2465,10 @@ 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;
+
 	if (repo_read_index_preload(the_repository, NULL, 0) < 0)
 		die(_("failed to read the index"));
 

builtin/grep.c

@@ -500,6 +500,10 @@ 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;
+
 	repo_read_gitmodules(subrepo, 0);
 
 	/*

builtin/mv.c

@@ -22,6 +22,7 @@
 #include "string-list.h"
 #include "parse-options.h"
 #include "read-cache-ll.h"
+#include "repo-settings.h"
 
 #include "setup.h"
 #include "strvec.h"
@@ -247,6 +248,10 @@ 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;
+
 	repo_hold_locked_index(the_repository, &lock_file, LOCK_DIE_ON_ERROR);
 	if (repo_read_index(the_repository) < 0)
 		die(_("index file corrupt"));

builtin/submodule--helper.c

@@ -16,6 +16,7 @@
 #include "read-cache.h"
 #include "setup.h"
 #include "sparse-index.h"
+#include "repo-settings.h"
 #include "submodule.h"
 #include "submodule-config.h"
 #include "string-list.h"
@@ -3832,5 +3833,9 @@ 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;
+
 	return fn(argc, argv, prefix, repo);
 }