Make it possible to use git tags/urls as target for old/new

[dkbast]

Is your feature request related to a problem? Please describe.
I would like to check for breaking changes between to git commits in a local git repository. A bonus would be if it was possible to point the tool to any hosted git repository as well (in addition to pub - eg. for self hosted packages).

Describe the solution you'd like
I would like to be able to pass a hash or similar identifier instead of a path, or attached to a path (e.g. --old #HEAD~2 or --old git:./#HEAD~2)

Describe alternatives you've considered
An alternative would be to create a wrapper script which creates a worktree ( git worktree add ../old-worktree <old-commit-hash> ) or an archive (git archive <old-commit> | tar xf - -C ../old-version) - and then clean it up afterwards. But since the tool already needs to be able to handle remote packages, I guess this could also be added to the cli.

Additional context
This would make this tool very useful for forensic debugging when trying to figure out where something broke as well as making it usable in other projects like serverpod where I think this could be used to help with versioning the generated clients (see serverpod/serverpod#2902 )