Add has_extension()

The little helper has_extension() documents through its name what we are
trying to do and makes sure we don't forget the underrun check.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>

local-fetch.c

@@ -43,8 +43,8 @@ static int setup_indices(void)
 		return -1;
 	while ((de = readdir(dir)) != NULL) {
 		int namelen = strlen(de->d_name);
-		if (namelen != 50 || 
-		    strcmp(de->d_name + namelen - 5, ".pack"))
+		if (namelen != 50 ||
+		    !has_extension(de->d_name, namelen, ".pack"))
 			continue;
 		get_sha1_hex(de->d_name + 5, sha1);
 		setup_index(sha1);