Hello.

I recently noticed that Bazaar's unified_diff() was emiting ---/+++ 
lines with a trailing whitespace if fromfiledate/tofiledate were the 
empty string. (Which was bad because a program to detect spurious 
trailing whitespace in diffs would flag them as errors.)

A patch [1] was committed recently in their tree. During the 
discussion, it was pointed out that the affected function was a copy 
of Python's own unified_diff() in difflib, which suffered the same 
problem.

  [1]: http://bazaar.launchpad.net/~bzr/bzr/trunk/revision/3923

I'm now attaching a patch to fix this issue in Python. It'd be great 
if you'd consider applying it. (It seems that the correct character to 
separate dates is a tab and not a space; if you feel changing this is 
breaking backwards compatibility, feel free to s/\t/ / in my patch.)

Thanks.