git-push: make git push --porcelain print "Done"

The script calling git push --porcelain --dry-run can see clearly from the
output if an update was rejected.  However, it will probably need to distinguish
this condition from the push failing for other reasons, such as the remote not
being reachable.

This patch modifies git push --porcelain to print "Done" after the rest of its
output unless any errors have occurred.  For the purpose of the "Done" line,
knowing a ref will be rejected in a --dry-run does not count as an error.
Actual rejections in non --dry-run pushes do count as errors.

Signed-off-by: Larry D'Anna <larry@elder-gods.org>
Acked-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

send-pack.h

@@ -4,6 +4,7 @@
 struct send_pack_args {
 	unsigned verbose:1,
 		quiet:1,
+		porcelain:1,
 		send_mirror:1,
 		force_update:1,
 		use_thin_pack:1,