Similar to what I recently did for SVN, this moves the Perforce revision

parsing (the thing that splits //depot/path#revision) into the diff

parser. This also will optimistically try to do that same parsing for

the modified file, in the case where we're posting a revision range of

already-submitted changesets.

Ran unit tests.
Posted a variety of changes across various revision ranges, both for

  submitted changes and pending changesets.