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