gpush: don't reject a plain tip revspec with --all

this is documented to work, and is actually useful. code which correctly
rejects the specification of ranges is already present further down.

amends b36c69ea.

Change-Id: I6ab153f2ccc601a6317a5cb96999e4f987e55562
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>