Add a new feature to the darc vmr diff command to enable comparison against the last backflow.

Context

• This idea is based on the user suggestion in #5088 and follows a similar pattern to the feature implemented in #5183.

Feature proposal

• Implement a default behaviour for darc vmr diff when called from a repo (not a VMR) which would diff against the VMR URI & sha found in the <Source> tag in the Version.Details.xml in the repo the command is called from.
• Should support both --name-only and full diff modes, with output and return code matching existing behavior.