push: give early feedback

There are many configuration variables that determine exactly what a
push does.  Give the user early feedback so that she has a chance to
abort if she doesn't mean to push those refspecs to that destination
like:

  $ git push
  # pushing refspecs 'master next' to ram (^C to abort)

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