Pull in cucumber tests from Hub

Since https://github.com/github/hub/compare/31a85e519e0a080dd10ce21339f806a9a0877766...master

Some tests are ignored either because they don't make sense or WIP
Merge remote-tracking branch 'origin/master' into pull_in_cucumber_tests

Conflicts:
	commands/pull_request.go
	git/git.go
Extract to local var refPrefix
Merge branch 'pull_in_cucumber_tests'