Fix KeyError: u'message' in SVN commits backend.
Review Request #6361 — Created Sept. 23, 2014 and submitted — Latest diff uploaded
Not every commit we get has a message, which can cause exceptions to be thrown
from the repository commits resource. We already had one fix for this sort of
error with the author fields. This change fixes it up to do the same for
message and author everywhere.
Ran unit tests.