Add a flag for ignoring whitespace in diffs.

I frequently find myself in a position where my editor has stripped a bunch of trailing spaces from a file, and I don't want those changes to show up in the review.  This change adds a new flag to post-review that tells diff to ignore whitespace.  The default behavior remains the same.

Currently this only works under Perforce (since I have limited time to test with SCM tools I never use), but presumably it could be generalized to most any other system.

Also, please forgive me if I've made any faux pas - this is my first time exposing my post-review hacks to the outside world :)

Generated diffs for a changelist (under Perforce) with lots of whitespace changes with and without the flag, and saw the expected results.