refspec: make @ a synonym of HEAD

refspec: make @ a synonym of HEAD

Since commit 9ba89f484e git learned how to push to a remote branch using
the source @, for example:

  git push origin @:master

However, if the right-hand side is missing, the push fails:

  git push origin @

It is obvious what is the desired behavior, and allowing the push makes
things more consistent.

Additionally, @:master now has the same semantics as HEAD:master.

Signed-off-by: Felipe Contreras <redacted>
Signed-off-by: Junio C Hamano <redacted>