grep: recurse in-process using 'struct repository'

Convert grep to use 'struct repository' which enables recursing into
submodules to be handled in-process.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git.c

@@ -392,7 +392,7 @@ static struct cmd_struct commands[] = {
 	{ "fsck-objects", cmd_fsck, RUN_SETUP },
 	{ "gc", cmd_gc, RUN_SETUP },
 	{ "get-tar-commit-id", cmd_get_tar_commit_id },
-	{ "grep", cmd_grep, RUN_SETUP_GENTLY | SUPPORT_SUPER_PREFIX },
+	{ "grep", cmd_grep, RUN_SETUP_GENTLY },
 	{ "hash-object", cmd_hash_object },
 	{ "help", cmd_help },
 	{ "index-pack", cmd_index_pack, RUN_SETUP_GENTLY },