refreshSubCommitsWithLimit now loads the sub-commits on the worker and
writes Model.SubCommits (and folds their authors into Model.Authors via
RefreshAuthors) inside an onUIThreadUnlessRepoChanged bounce.
SubCommitsMutex and AuthorsMutex are left in place: the former is shared
with setSubCommits, the latter with the commits refresh's RefreshAuthors
call, so both come out only once those other writers are on the UI thread
too.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>