transport-helper: improve push messages

If there's already a remote-helper tracking ref, we can fetch the
SHA-1 to report proper push messages (as opposed to always reporting
[new branch]).

The remote-helper currently can specify the old SHA-1 to avoid this
problem, but there's no point in forcing all remote-helpers to be aware
of git commit ids; they should be able to be agnostic of them.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Felipe Contreras

t/t5801-remote-helpers.sh

@@ -166,4 +166,18 @@ test_expect_success 'push ref with existing object' '
 	compare_refs local dup server dup
 '
 
+test_expect_success 'push messages' '
+	(cd local &&
+	git checkout -b new_branch master &&
+	echo new >>file &&
+	git commit -a -m new &&
+	git push origin new_branch &&
+	git fetch origin &&
+	echo new >>file &&
+	git commit -a -m new &&
+	git push origin new_branch 2> msg &&
+	! grep "\[new branch\]" msg
+	)
+'
+
 test_done

transport-helper.c

@@ -801,6 +801,7 @@ static int push_refs_with_export(struct transport *transport,
 		if (private && !get_sha1(private, sha1)) {
 			strbuf_addf(&buf, "^%s", private);
 			string_list_append(&revlist_args, strbuf_detach(&buf, NULL));
+			hashcpy(ref->old_sha1, sha1);
 		}
 		free(private);