(bug) scanDiff fails on force pushes

[coopernetes]

Describe the bug
It is common for developers to rebase to modify commits such as add missing DCO or commit signing. GitProxy does not handle this scenario properly and will generate opaque or unclear errors due to failures in the diffing steps. Often, this leads to developers having to start over from scratch with a new branch and/or PR which introduces unnecessary toil.

To Reproduce
Will add later once I've gathered some additional logs. To reproduce, simply amend an existing (pushed) commit and attempt to use git push proxy --force.

Expected behavior
GitProxy can handle pushes made forcefully and get the appropriate diffs from the upstream branches.

[kriswest]

We've noted that merging main (e.g. to resolve conflicts that have arrive since you raised a PR) will incorporate incoming commits into diffs as the diff is based on the branch's base I think, rather than the current HEAD of main - as is also the case with both GitHub and GitLab). That adds unnecessary weight to the diff. That can be avoided with a rebase - although you will need re-review the PR's commits as their IDs change on rebase but that may prove to be the lesser evil when lots is being merged into main or PRs hang out for while awaiting review). A force push will be needed to support rebase.