gitstats: Restructured the commit and diff module so that diff depends on commit

gitstats: Restructured the commit and diff module so that diff depends on commit

After finding a buggy 'getCommitDiff' in diff.py, while we
already have a very well functioning one in commit.py, it
made sense to make diff.py depend on commit.py. Previously
commit.py used one method from diff.py (being findReverts)
so that diff.py could not depend on commit.py (since that
would create a circular dependency). To fix this the
findReverts function was moved to diff.py (together with
'commit -v' => 'diff -r').