Enable PR merge squashing

[nyurik]

The automated merge-queue system keeps all original revisions of the PR. This polutes git history without much value -- if needed, the individual PR steps can still be viewed in the PR itself. Can we enable squashing somehow?

[michaelkirk]

Semantic commits are wonderful. For example you can separate out small refactors from feature changes.

A squashed commit is better than a ton of garbage commits.

I don't want to consider github.com as a source of record any more than necessary.

My preference is that it be possible, but not required, to squash commits.

[nyurik]

as long as we can use it (possibly by default unless explicitly ask not to), works for me. The problem is that ever since we enabled merge pipeline on this repo, the dropdown for the "merge" button no longer has any options...