Lots of failures on push-to-pull-request-branch

[dsyme]

fsprojects/FSharp.Formatting#993 contains a lod of failures to push-to-pull-request-branch

Some of these may be because the PR is in a forked repo - which we should fail much earlier more gracefully

Some may be because the PR being updated actually had merge conflicts already - in which case it may be understandable that it gets harder to work on it.

Will investigate more

[dsyme]

We've hopefully made progress on this in #18175