Stop using the untracked local opam file on opam pin ./local-vcs

[rjbou]

fix #6408
I'm not fully satisfied with that solution...

This PR changes the behaviour of opam pin add ./local-vcs which would take the opam files from the VCS repository even if they had untracked changes. This behaviour exists since opam 2.1 (see #4300) but opam 2.0 didn't have the same behaviour.
The addition of a note about untracked changes can be added separately and is tracked by #6414

[kit-ty-kate]

After lengthy reviewing and tinkering i wasn't satisfied with the solution either so i came up with a different proposal, which while a bit radical i strongly believe to be a positive change for users:

Currently pinning a local vcs directory will override the opam files with whatever the version in the working directory is, regardless of whether or not --working-dir is given. I think this behaviour is too surprising to users. If they wish to use the working directory version then they can pass --working-dir explicitly. This also has the side-effect to simplify our code quite a lot which is always nice.

I've pushed the proposal as the fixup proposal commit above. The parent fixup commit implements a couple of cleanups (probably mostly redundant if the proposal is to be accepted), and i've also added a commit at the top to change the return type of OpamRepository.revision which shouldn't have had the version type.

What do you think?