SCMTool.get_file() and SCMTool.file_exists() gained the addition of

a base_commit_id many releases ago, and when that happened we needed

to deprecate the old signature and warn if used, but continue to support

it.
This change removes support for the deprecated signatures, which is long

overdue but also helps with Python 3 compatibility (as the function we

were using to check was also deprecated in Python 3).

Unit tests pass.