Support git style diffs for HGTool if they are part of an HG Changeset.

Review Request #1174 — Created Oct. 20, 2009 and discarded — Latest diff uploaded

Review Board
Issue #472 says that "HGTool does not support git style diff" because "git-style diffs from Hg don't include revision information."  However if that git-style if came from an 'hg export' command then it does have the information in the header.

This code parses it.

Hold on a second. This doesn't handle all the cases.  I'll have to work on this a little more.
Uploaded a diff generated by hg export, hg export --git, hg diff.  All succeeded.
Tried to upload a diff generate by hg diff --git.  Still failed.