CI version bump check can get confused when PR is behind master

[ehuss]

The check-version-bump check generates an error in the following case:

1. Contributor creates a branch.
2. Some PR merges that modifies a crate, for example bumping cargo-credential's version number.
3. Contributor submits their PR, where its base is before step 2 is merged.

This causes an error like:

   BumpCheck base commit `819fa73b2dd86e32bf01a7317aec0fa0a447c871`
   BumpCheck head commit `c9262bd58021cd3bcca101f68bc3a9efbb57cb70`
   BumpCheck compare against `crates-io`
    Updating crates.io index
   BumpCheck compare against `9c4383fb55986096b414d98125421ab87b5fd642`
error: Detected changes in these crates but no version bump found:
  [email protected]
  [email protected]

I think the issue is that the diff is not using a "symmetric difference" between base and head. In the example above, base is the latest master (with the changes to cargo-credential) and head is the tip of my branch (which is missing those changes). The check is doing the equivalent of git diff 819fa73b2dd86e32bf01a7317aec0fa0a447c871 c9262bd58021cd3bcca101f68bc3a9efbb57cb70, which pulls in all changes that have happened on master since the branch was created. I think a solution is to use a symmetric diff (with git's ... syntax), but AFAIK libgit2 does not support that. So I'm not sure how to solve this.

[weihanglo]

How about using git when it is available, and libgit2 as a fallback?

I think in most cases git is there and we're good.

[ehuss]

I think it would be fine to unconditionally use the git CLI. I think it is safe to say anyone running this will have it installed.