Try to make git optional
Try to make git optional

The tests will pick any of git/hg/bzr/svn, whichever is available.
You can force the selection by setting FORCE_TEST_VCS.

Slight downside to this:
- TestCheckManifest.test_missing_source_files fails with bzr/hg
- TestCheckManifest.test_relative_{pathname,python} fail with svn

I'll now investigate if those are real bugs.

I wonder if I should add a Travis matrix for forcing all the VCS
systems?

mauritsvanrees/check-manifest,mgedmin/check-manifest