Add SCMClient.parse_revision_spec and implement/use it in GitClient.
As part of redoing the revision-range syntax for rbt post, I've decided to

add a method to SCMClient that can take in an array of revisions and "parse"

them, returning a dictionary with 'base', 'tip', and optional 'parent_base'

keys. This method does all the hard work of separating out the passed-in

revisions and detecting whether or not a parent diff is required.
This method is currently only implemented for the GitClient. I've added a bunch

of unit tests to make sure it returns the right revisions, and also refactored

the diff generation code to use it for both the diff and

diff_between_revisions methods.

Ran unit tests.
Posted this change with the new code.