push: do not turn --delete '' into a matching push

When we added a syntax sugar "git push remote --delete <ref>" to
"git push" as a synonym to the canonical "git push remote :<ref>"
syntax at f517f1f2 (builtin-push: add --delete as syntactic sugar
for :foo, 2009-12-30), we weren't careful enough to make sure that
<ref> is not empty.

Blindly rewriting "--delete <ref>" to ":<ref>" means that an empty
string <ref> results in refspec ":", which is the syntax to ask for
"matching" push that does not delete anything.

Worse yet, if there were matching refs that can be fast-forwarded,
they would have been published prematurely, even if the user feels
that they are not ready yet to be pushed out, which would be a real
disaster.

Noticed-by: Tilman Vogel <tilman.vogel@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/push.c

@@ -115,7 +115,7 @@ static void set_refspecs(const char **refs, int nr, const char *repo)
 			else
 				refspec_appendf(&rs, "refs/tags/%s", ref);
 		} else if (deleterefs) {
-			if (strchr(ref, ':'))
+			if (strchr(ref, ':') || !*ref)
 				die(_("--delete only accepts plain target ref names"));
 			refspec_appendf(&rs, ":%s", ref);
 		} else if (!strchr(ref, ':')) {

t/t5516-fetch-push.sh

@@ -818,6 +818,11 @@ test_expect_success 'push --delete refuses src:dest refspecs' '
 	test_must_fail git push testrepo --delete master:foo
 '
 
+test_expect_success 'push --delete refuses empty string' '
+	mk_test testrepo heads/master &&
+	test_must_fail git push testrepo --delete ""
+'
+
 test_expect_success 'warn on push to HEAD of non-bare repository' '
 	mk_test testrepo heads/master &&
 	(