pack-write: simplify index_pack_lockfile using skip_prefix() and xstr…

…fmt()

Get rid of magic string length constants by using skip_prefix() instead
of memcmp() and use xstrfmt() for building a string instead of a
PATH_MAX-sized buffer, snprintf() and xstrdup().

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-write.c

@@ -288,13 +288,12 @@ char *index_pack_lockfile(int ip_out)
 	 * case, we need it to remove the corresponding .keep file
 	 * later on.  If we don't get that then tough luck with it.
 	 */
-	if (read_in_full(ip_out, packname, 46) == 46 && packname[45] == '\n' &&
-	    memcmp(packname, "keep\t", 5) == 0) {
-		char path[PATH_MAX];
+	if (read_in_full(ip_out, packname, 46) == 46 && packname[45] == '\n') {
+		const char *name;
 		packname[45] = 0;
-		snprintf(path, sizeof(path), "%s/pack/pack-%s.keep",
-			 get_object_directory(), packname + 5);
-		return xstrdup(path);
+		if (skip_prefix(packname, "keep\t", &name))
+			return xstrfmt("%s/pack/pack-%s.keep",
+				       get_object_directory(), name);
 	}
 	return NULL;
 }