Implement revision parsing for Perforce.
This change implements PerforceClient.parse_revision_spec. This can take in

zero, one, or two arguments.
With zero arguments, this will return the 'default' changelist. With one, it

can be a changelist that's either pending, shelved, or submitted. In all three

of those cases, the changes that will be posted will be the changes introduced

in that changelist. With two arguments, the first argument must be a submitted

changelist, and the second can be any of pending, shelved, or submitted.
This is not yet used for diffs, since handling submitted and shelved changesets

will take additional work.

Ran unit tests.