feeds: fix failure getting feed for Git repos (Issue #372)

GitChangeset.diff() did
return ''.join(self.repository.get_diff(...))
even though get_diff returned a string. It worked, but was unnecessary and
inefficient.

That fails in py3: get_diff returns bytes ... and iterating doesn't give
characters but integers and we would get:
TypeError: sequence item 0: expected a bytes-like object, int found

Fixed by dropping the unnecessary iteration and joining.

This function is only used for feeds statistics.