Better diff argument check for git-review

This is silly, but I want `git review foo --diff` and `git review --diff
foo` to both work. The case statement pattern seems like the easiest way
to allow -d and --diff.

I played with getopts for a while and tried looping through the params
with shift but my bash script skills were not up to the task.

Two case statements? I can kludge that no problem. :-)