Prevent a unicode error on UTF-8 in commit metadata.

Having utf-8 in the commit metada (e.g author, commit message, file
names) triggered a decoding error:

"UnicodeDecodeError: 'ascii' codec can't decode byte 0xd7 in position 42: ordinal not in range(128)"

This is because the __unicode__ method of the Commit class, doesn't
actually return an object of type 'unicode' although it should.