Reuse results of try in merge, if possible

(I assume the PR is approved)

If try has finished successfully and the branch hasn't moved, merge should just do the merge, without kicking off another CI run.

If try is running and the branch hasn't moved, merge should wait until the try run finishes and either merge or fail.

Either way, basically, we want to promote the try run and avoid kicking off a new CI run for staging.

To maintain correctness, this requires that the staging is expressible try + on-success, and that Mergit knows about and enforces this, which may be out of scope.

Start by tracing the existing try and merge flows and how GitHub Checks runs are represented; the issue names no files or tests. Done means merge can promote a successful try run, wait for an in-progress try when the branch is unchanged, and enforce the stated correctness condition or document its scope.

Written by the indexing model from the issue text.