Add a test SCMTool with a DiffX parser.
Review Request #12065 — Created Feb. 20, 2022 and submitted
TestToolDiffX, which is a simple Git-based SCMTool that uses
a DiffX parser by default. This allows us to more easily test operations
involving a repository that uses DiffX files.
As of this change, nothing currently uses this tool, but upcoming
changes will begin to make use of it.
Unit tests pass.
Successfully made use of this with an upcoming change.