Remove hard dependency on HG for tests.
The tests for the commits and branches resources used a mercurial repository to

test for repos that didn't support the calls, but didn't check to see if hg was

installed. This resulted in tracebacks for these two tests. I've changed this

to use the CVS tool for these tests instead, which doesn't have any external

dependencies.
There's also one test in the diffviewer code that just should be skipped when

mercurial isn't installed.

Ran unit tests.