After repository commit status has been introduced on dashaboard, the
most top SQL comes from `GetLatestCommitStatusForPairs`.
This PR adds a cache for the repository's default branch's latest
combined commit status. When a new commit status updated, the cache will
be marked as invalid.
<img width="998" alt="image"
src="https://github.com/go-gitea/gitea/assets/81045/76759de7-3a83-4d54-8571-278f5422aed3">
