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 :)