Use git show-ref instead of git branch for better future-proofness.

Based on original patch by Anrs Hu.
Combine Git get_tag_revs and get_branch_revs into single get_refs method.
Merge branch 'develop' into bugfix-git183-v2

* develop:
  add back in old tox.ini
  add back test lost in the restructure
  more functional/unit tests restructure
  break up tests dir into data, unit tests, and functional tests
Merge pull request #967 from carljm/bugfix-git183-v2

Use git show-ref instead of git branch for better future-proofness.