clone: prevent --reference to a shallow repository

If we borrow objects from another repository, we should also pay
attention to their $GIT_DIR/shallow (and even info/grafts). But
current alternates code does not.

Reject alternate repos that are shallow because we do not do it
right. In future the alternate code may be updated to check
$GIT_DIR/shallow properly so that this restriction could be lifted.

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

builtin/clone.c

@@ -252,6 +252,12 @@ static int add_one_reference(struct string_list_item *item, void *cb_data)
 		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);
+
+	if (!access(mkpath("%s/info/grafts", ref_git), F_OK))
+		die(_("reference repository '%s' is grafted"), item->string);
+
 	strbuf_addf(&alternate, "%s/objects", ref_git);
 	add_to_alternates_file(alternate.buf);
 	strbuf_release(&alternate);