All SCMTool implementations should now take keyword arguments and a

base_commit_id argument. All internal implementations have been

converted and third party tools will cause a deprecation warning if

they do not have the proper method signature.
The HgTool now takes advantage of this `base_commit_id` argument when

fetching files (since hg git diffs do not include index lines which

we use to determine what revisions to fetch). If `base_commit_id` is

provided it will override anything parsed from the diff.
The same applies to SCMTool.file_exists()

New tests, unit tests pass, posted hg diff now uses the provided base_commit_id.