Merge branch 'jc/push-refmap'

Merge branch 'jc/push-refmap'

Make "git push origin master" update the same ref that would be
updated by our 'master' when "git push origin" (no refspecs) is run
while the 'master' branch is checked out, which makes "git push"
more symmetric to "git fetch" and more usable for the triangular
workflow.

* jc/push-refmap:
  push: also use "upstream" mapping when pushing a single ref
  push: use remote.$name.push as a refmap
  builtin/push.c: use strbuf instead of manual allocation