Enables the Mercurial diff parser to handle file paths with spaces.

I don't know Python, and was therefore "coding by Google", so please expect to find some issues.

Update: diff #2 comes with proposed changes generously applied by bboissin (via http://code.google.com/p/reviewboard/issues/detail?id=541)

Tested locally on Debian with various changesets that do and do not include paths with spaces.