merges to branches with active/pending/wip reviews against them should re-generate diff

[Drew Freiberger]

It would be helpful if Merge Proposal diffs were updated if the target branch for the MP is updated. Essentially, it would be helpful to know if a stale Merge proposal needs to be rebased by looking at the diff/status of the MP.

For instance, if I propose a branch to merge to master in October, and there are three commits in November, and I return to my proposal for the October branch and am ready to merge in December, the diff is not updated to show that there are conflicts with changes made in November.

Example MR:
https://code.launchpad.net/~llama-charmers/charm-prometheus-ceph-exporter/+git/charm-prometheus-ceph-exporter/+merge/391620

The MR shows a clean diff, but master has moved on since that commit and a 3 way merge/rebase was required, but was not visible via the launchpad review interface.