This patch removes the above mentioned fields because we will now be

tracking per-commit FileDiffs and cumulative (i.e., a cumulative diff of

the entire commit series) FileDiffs in the DiffSet.files field, and

the file_count field will no longer be of any use in that case.
Since the mixin defining the file_count would only now be providing a

utility method, the mixin has been removed and the method has been moved

into a free function which both DiffCommit.get_total_line_counts and

DiffSet.get_total_line_counts call into.

Ran unit tests.