Fix diff generation with unedited files in Perforce.

If you make a perforce-backed code review with an unedited (but opened)
file then ReviewBoard chokes on the empty diff content. Based on the
recentness of this issue I suspect that it's regression for us from
upgrading ReviewBoard from 1.5 to 1.6.

This issue has been discussed by others on...

http://code.google.com/p/reviewboard/issues/detail?id=2754

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Steps to repro:

1. p4 edit some/file/with/stuff
2. run post-review

Expected:

Post-review should error out if you don't have any edits. If you have
other modifications then it should just make a code review with that.

Actual:

Post-review always succeeds, but the unedited file renders as a
stacktrace in ReviewBoard.

Fixes bug #2754
Reviewed at http://reviews.reviewboard.org/r/3944/