Tests for get_vcs_files()

Actually invokes git/svn/hg/bzr in a temporary directory, so you need to
have all of those installed.

Highlights two bugs with svn support:

1. If you svn add but do not svn commit, get_svn_files() will not notice
   the files you added.

2. If you svn add and svn commit, but do not svn up, get_svn_files()
   will not notice the files you added.

There was already an XXX about that.