Tags: teach git_tag_list not to include the 'refs/tags/' prefix

Since quite a while now, git_branch_foreach has learnt to list branches
without the 'refs/heads/' or 'refs/remotes' prefixes.
This patch teaches git_tag_list to do the same for listing tags.

src/tag.c

@@ -408,7 +408,7 @@ static int tag_list_cb(const char *tag_name, void *payload)
 
 	filter = (tag_filter_data *)payload;
 	if (!*filter->pattern || p_fnmatch(filter->pattern, tag_name + GIT_REFS_TAGS_DIR_LEN, 0) == 0)
-		return git_vector_insert(filter->taglist, git__strdup(tag_name));
+		return git_vector_insert(filter->taglist, git__strdup(tag_name + GIT_REFS_TAGS_DIR_LEN));
 
 	return 0;
 }