Merge branch 'ps/includeif-onbranch-cornercase-fix'

"git --git-dir=nowhere cmd" failed to properly notice that it
wasn't in any repository while processing includeIf.onbranch
configuration and instead crashed.

* ps/includeif-onbranch-cornercase-fix:
  config: fix evaluating "onbranch" with nonexistent git dir
  t1305: exercise edge cases of "onbranch" includes