sha1_name: implement finding @{push}

Try this now: configure your current branch's pushremote to push to
"refs/heads/*:refs/heads/rr/*".  Now, type 'git show @{p}'.  Voila!

It currently only works when:

1. remote.<name>.push is explicitly specified.
2. There is a pattern to match (*).

Proof-of-concept only.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>