push: don't push the volatile HEAD with current

Since a push does not lock the "HEAD" ref, there is no guarantee that it
will remain the same for the entire operation of the push.  Practically,
this means that with push.default set to current:

  # on branch push-current-head
  $ git push
  # on another terminal
  $ git checkout master
  # return to the first terminal
  # the push tried to push master!

Avoid this confusion by adding the branch that HEAD resolves to as the
refspec to push (as opposed to the literal "HEAD").  With this change,
the output of the push changes subtly from:

  $ git push
  ...
   * [new branch]      HEAD -> push-current-head

to:

  $ git push
  ...
   * [new branch]      push-current-head -> push-current-head

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