fetchdiff.py: support specifying diff revision

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add fetch-diff

Add fetch-diff, a tool to grab raw diffs from Review Board using the
same authentication and server-guessing as post-review. For now, only
grabs the latest diff.
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(end commit log)

Here is a little something to motivate "library-izing" rbtools. I was asked to run an automated build against a review request (our internal Review Board), and I thought, "wouldn't it be great if I could feed the builder a request number instead of making a patch and pointing it at the full path to the patch?". Well, now I can :-).

It is probably more interesting to diff the new script against postreview.py. The only altered functions (I outright dropped a lot of stuff) could be reverted (i.e. use the postreview.py versions in new, shared code); they just need some options to be set to empty instead, to fix missing variable errors.

Please let me know if the 'git format-patch' patches are wanted (i.e. to preserve the history, tiny though it is).

Works against our internal server, using both git-svn and svn repositories to find the server information; both '#' and '#:#' request formats tested. Also this was posted using postreview.py including http://reviews.reviewboard.org/r/1465 :-).