builtin/clone.c: detect a clone starting at a tag correctly

31b808a (clone --single: limit the fetch refspec to fetched branch,
2012-09-20) tried to see if the given "branch" to follow is actually
a tag at the remote repository by checking with "refs/tags/" but it
incorrectly used strstr(3); it is actively wrong to treat a "branch"
"refs/heads/refs/tags/foo" and use the logic for the "refs/tags/"
ref hierarchy.  What the code really wanted to do is to see if it
starts with "refs/tags/".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/clone.c

@@ -695,7 +695,7 @@ static void write_refspec_config(const char* src_ref_prefix,
 	if (option_mirror || !option_bare) {
 		if (option_single_branch && !option_mirror) {
 			if (option_branch) {
-				if (strstr(our_head_points_at->name, "refs/tags/"))
+				if (starts_with(our_head_points_at->name, "refs/tags/"))
 					strbuf_addf(&value, "+%s:%s", our_head_points_at->name,
 						our_head_points_at->name);
 				else