Fix the affinity of the "pdiff_required" value.

When using --revision-range with git, we determine whether or not we need to
create a parent diff by checking if the upstream branch contains the source
revision. The result of this check went into a variable called
"pdiff_required", but the value of this variable was truthy if a parent diff is
*not* required. I've negated the assignment expression to flip this back to
mean what it says.

Testing done:
Posted a review request with --revision-range that uses a parent diff.

Reviewed at https://reviews.reviewboard.org/r/5102/
Use md5 hashes of hg diffs instead of including the full text.

This shortens our test file a bit by checking that the hash of the diff matches
what we expect, rather than just comparing long text strings.

Testing done: Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/5114/
Use md5 hashes of perforce and bzr diffs instead of including the full text.

This shortens our test file a bit by checking that the hash of the diff matches
what we expect, rather than just comparing long text strings.

Testing done: Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/5115/
Allow third-party packages to register new rbt commands.

rbt will now scan for third-party commands that are registered
for the 'rbtools_commands' entrypoint.

Testing done: Ran unit tests, manual testing with a third-party
command provider.

Reviewed at https://reviews.reviewboard.org/r/5046/.
Do some trivial cleanup of the _hgcmd method.

Not all these parameters were used, and the name itself was kind of annoying.
Simple cleanup.

Testing done: Ran unit tests

Reviewed at https://reviews.reviewboard.org/r/5117/
Do some simple refactoring of the post command.

This change does some cleanup that struck me as I was reading through the post
implementation. I've moved the score matching out into a new utils file, and
rearranged/commented the argument evaluation a little bit.

Testing done:
* Ran unit tests
* Posted a change

Reviewed at https://reviews.reviewboard.org/r/5118/