add --porcelain option to git-push

If --porcelain is used git-push will produce machine-readable output.  The
output status line for each ref will be tab-separated and sent to stdout instead
of stderr.  The full symbolic names of the refs will be given.  For example

$ git push --dry-run --porcelain master :foobar 2>/dev/null \
| perl -pe 's/\t/ TAB /g'

= TAB refs/heads/master:refs/heads/master TAB [up to date]
- TAB :refs/heads/foobar TAB [deleted]

Signed-off-by: Larry D'Anna <larry@elder-gods.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>