Allow @ shortcut with git-push

Until now, HEAD could be used with git-push to push the current
branch. Now @ is a shortcut to HEAD, but it didn't work when
pushing branches. It fails with:

  fatal: remote part of refspec is not a valid name in @

Reinterpret all branch names from argv in order for @ to be used
when pushing branches.

Signed-off-by: Tomo Krajina <tkrajina@gmail.com>