RFC: Fail fast on AppVeyor for repeated commits to the same PR

To cut down on queue times. Opening this to test a few things.

AppVeyor runs:
```
git fetch -q origin +refs/pull/$APPVEYOR_PULL_REQUEST_NUMBER/merge:
git checkout -qf FETCH_HEAD
```
at the start of a PR build. For queued builds to master we would need some more complicated logic but we might want to build every commit to master separately anyway.

The simple version of this (just checking `$APPVEYOR_REPO_COMMIT` against `git rev-parse HEAD`) won't work if there's a new commit to master between the time the build gets queued and the time the build starts running, regardless of whether there are any newer commits to the PR branch.