Implement posting for shelved perforce changes.
The perforce shelf is kind of like a shared "git stash". When you shelve a

change, it copies the data from your working directory over to the server. What

I didn't expect is that it also leaves your working directory the same, with

the same open files (which can then be reverted).
We can get the file list for a shelved change via `p4 files //...@=CLN`, and

the file contents via `p4 print //path/to/file@=CLN`. This lets us use a very

similar implementation to pending changes, with the exception that the "new"

files are fetched from the repository instead of just using the working copy.
The perforce documentation is full of lies. Among other fantastical claims, it

asserts that shelved changes have a 'Status' field of 'shelved', when in

practice, they're still 'pending'. I've made changes to parse_revision_spec to

treat shelved changes the same as pending ones, and then made some changes to

the pending diff code path to look on the shelf if there are no open files in

the changeset.
The one major limitation that I wasn't able to work around is that when you

shelve a change containing file moves, there doesn't seem to be any way to know

which "move/add" corresponds to which "move/delete". The usual ways of doing

this (looking for "movedFile" in fstat, or looking in the filelog) don't work

for shelved changesets. I suspect that this is just an oversight that will be

corrected in the course of time, but for now, I've made it so that when you

post such a change, you'll see an old-school deleted file and an added file,

even when the Review Board server supports moved files.

Posted a variety of shelved changes with different types of changes contained

  within.
Ran rbt diff N with a shelved changeset, which is what prompted the addition

  of the options to that command.
Ran unit tests.