gc: replace local buffer with git_path

We probe the "17/" loose object directory for auto-gc, and
use a local buffer to format the path. We can just use
git_path() for this. It handles paths of any length
(reducing our error handling). And because we feed the
result straight to a system call, we can just use the static
variant.

Note that git_path also knows the string "objects/" is
special, and will replace it with git_object_directory()
when necessary.

Another alternative would be to use sha1_file_name() for the
pretend object "170000...", but that ends up being more
hassle for no gain, as we have to truncate the final path
component.

Signed-off-by: Jeff King <peff@peff.net>

builtin/gc.c

@@ -135,8 +135,6 @@ static int too_many_loose_objects(void)
 	 * distributed, we can check only one and get a reasonable
 	 * estimate.
 	 */
-	char path[PATH_MAX];
-	const char *objdir = get_object_directory();
 	DIR *dir;
 	struct dirent *ent;
 	int auto_threshold;
@@ -146,11 +144,7 @@ static int too_many_loose_objects(void)
 	if (gc_auto_threshold <= 0)
 		return 0;
 
-	if (sizeof(path) <= snprintf(path, sizeof(path), "%s/17", objdir)) {
-		warning(_("insanely long object directory %.*s"), 50, objdir);
-		return 0;
-	}
-	dir = opendir(path);
+	dir = opendir(git_path("objects/17"));
 	if (!dir)
 		return 0;