clone: better error when --reference is a linked checkout

Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/clone.c

@@ -294,9 +294,14 @@ static int add_one_reference(struct string_list_item *item, void *cb_data)
 		char *ref_git_git = mkpathdup("%s/.git", ref_git);
 		free(ref_git);
 		ref_git = ref_git_git;
-	} else if (!is_directory(mkpath("%s/objects", ref_git)))
+	} else if (!is_directory(mkpath("%s/objects", ref_git))) {
+		struct strbuf sb = STRBUF_INIT;
+		if (get_common_dir(&sb, ref_git))
+			die(_("reference repository '%s' as a linked checkout is not supported yet."),
+			    item->string);
 		die(_("reference repository '%s' is not a local repository."),
 		    item->string);
+	}
 
 	if (!access(mkpath("%s/shallow", ref_git), F_OK))
 		die(_("reference repository '%s' is shallow"), item->string);