Add the `darc vmr diff` command

[premun]

Context

We will need a command that will tell us the current delta between a repo and a VMR. We will have to think about what we will compare and how. For example:

• We want to just compare branches but those can have pending PRs open still.
• Or we can compare a commit with the matching counterpart commit that was synchronized.

I think we have to try out what it returns to see what is useful. We have to also consider not diffing some files (like version files) if that is not useful.

It would be nice if it was intuitive but we would not have to pass too many options:

• Example: if it's called from within the VMR folder, it can only accept mapping name and that might be enough
• If it's called from within a repo, it might just have all the information it needs
• Or maybe it's just easier to always supply the mapping name and it will find all information regardless of where it's called from
• Or maybe it's easier to be explicit and always supply from and to, so you could have darc vmr diff repo:REF1..vmr:REF2 or darc vmr diff vmr:REF1..repo:REF2

So please think about it and come up with a UX proposal.

Goal

• Create a new command darc vmr diff
• The command has to either output the diff to console or have an --output [PATH] option

[dkurepa]

I think the only input this command needs is --repo and --branch. Source mapping won't be enough if we have multiple VMR branches, for example main and previews. With these two, we can find which VMR branch we want to compare against