Change how commit message parsing and diff calls work.

The SCMClients had some confusing responsibilities where they attempted
to handle the guessed summary/description logic in the diff methods,
getting and setting data in the options object. This logic had no
business being in the diff method, or in SCMClients for that matter, and
should have exclusively been in rbt post.

On top of that, we had some variations in how the commit summary and
description were fetched and processed in the different SCMClients. This
led to differences in behavior and subtle bugs. For example, Mercurial
was grabbing the summary from the wrong end of a commit range, and
Bazaar was providing extraneous information for the descriptions.

Now, instead of extract_summary() and extract_description() methods,
SCMClients only need to implement get_raw_commit_message(). This returns
the full set of commit messages for the provided revisions. The base
SCMClient class then provides a get_commit_message() that calls that and
splits the result into a summary and a description.

The description also no longer includes the summary line, preventing the
first line of the Description field from mirroring the Summary field
unnecessarily.

rbt post is then responsible for calling this when it needs to generate
the guessed content or match an existing review request in order to
update it.

Since both get_commit_message() and diff() need parsed revisions, it no
longer makes sense for diff() to call parse_revision_spec(). That's now
the job of the caller, which results in diff() being just a bit simpler.

At this point, the get_diff() utility function is no longer necessary
either, so it's been removed.

Now that `rbt post` owns more of the logic, it can also be smarter about
when things are called. Previously, if using -u, we'd parse the
revision spec twice (once in diff() and once when generating the
summary/description for matching).

We also would make at least two calls to get the summary and description
(more, if using -g and the description was blank).

`rbt post` now caches the parsed revision spec, and the commit message,
preventing us from having to run as many costly commands.

This all results in a cleaner design with less responsibility in
SCMClient, fewer usages of the options object, and faster posting.