Fixes broken diffs posted by post-review in subdirs of a git svn work-tree
Probably not elegant, but it's simple and works.

There is a --relative option to the git diff command since 1.5.5.x, but many default installation of git for linux is still stuck at 1.5.2.x, so the cd approach is used. However, I want to point out that this is not bullet proof as git can have GIT_DIR defined elsewhere (outside the work tree). But my patch would work for typical installations. Suggestions are welcome, of course.