Allow user to specify local VMR clone to use for `darc vmr resolve-conflict`

[jkoritzinsky]

As part of the darc vmr resolve-conflict command (and other commands for the VMR I assume), darc will clone a copy of the VMR to a temp directory. For those of us who already have a copy of the VMR locally, it would be nice to be able to reuse an existing clone and have darc just fetch the updates to that existing clone. This saves disc space and should be faster/more efficient for workflows when needed.

This command does provide a --vmr option, but it does not stop darc from cloning the VMR to a temp directory.

[premun]

This sounds like a bug in darc. Providing a custom --vmr should make the command use it. I guess we don't register it properly.

[dkurepa]

we've added a new --source-repo parameter that can be used for both Forward and Backflows (repo in FF, vmr in BF).
The --vmr parameter also works instead of the --source-repo for backflows too now