Merge branch 'jk/rev-parse-local-env-vars'

Merge branch 'jk/rev-parse-local-env-vars'

The "--local-env-vars" and "--resolve-git-dir" options of "git
rev-parse" failed to work outside a repository when the command's
option parsing was rewritten in 1.8.5 era.

* jk/rev-parse-local-env-vars:
  rev-parse: let some options run outside repository
  t1515: add tests for rev-parse out-of-repo helpers