Merge branch 'jc/push-default-explicit'

A lazy "git push" without refspec did not internally use a fully
specified refspec to perform 'current', 'simple', or 'upstream'
push, causing unnecessary "ambiguous ref" errors.

* jc/push-default-explicit:
push: test pushing ambiguously named branches
push: do not use potentially ambiguous default refspec

diff --cc builtin/push.c
Simple merge