push: fail early with detached HEAD and current

push: fail early with detached HEAD and current

Setting push.default to current adds the refspec "HEAD" for the
transport layer to handle.  If "HEAD" doesn't resolve to a branch (and
since no refspec rhs is specified), the push fails after some time with
a cryptic error message:

  $ git push
  error: unable to push to unqualified destination: HEAD
  The destination refspec neither matches an existing ref on the remote nor
  begins with refs/, and we are unable to guess a prefix based on the source ref.
  error: failed to push some refs to 'git@github.com:artagnon/git'

Fail early with a nicer error message:

  $ git push
  fatal: You are not currently on a branch.
  To push the history leading to the current (detached HEAD)
  state now, use

    git push ram HEAD:<name-of-remote-branch>

Just like in the upstream and simple cases.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>