[rocketraman]

Clean up merged remote branches

Each remote's heading in Remote branches gets Clean up N merged. It deletes every merged branch shown for that remote, including squash-merged ones, and skips integration branches. A confirmation lists the branches first.

• It's scoped per remote: in a fork, sweeping your own copy and sweeping the project's remote are separate decisions.
• Each delete is leased on the tip from the last fetch, so a branch someone has pushed to since is skipped, not deleted with their commits.
• A branch already deleted on the remote counts as deleted, and its stale local copy is dropped.
• It's one ls-remote plus chunked pushes per remote, not a push per branch.

Remote branch list: loading indicator and speed

On a repo with ~250 remote branches the list took ~22s to appear and showed "No remote branches" until then. It now shows Loading…, and takes ~1.7s cold and ~25ms when you come back to the repo.

• Cause: the squash-merge check ran git log -p … -- <every file the branch changed>. git matches each pathspec against every commit it walks, which took 5–7s for a single branch of a few thousand files; the same log with no pathspec takes 40ms. Candidate commits now come from a --name-only log with no pathspec, and only those few are diffed. Results matched the old algorithm on all 249 branches of the test repo.
• The answers are cached by (branch tip, mainline tip).
• "In mainline's history" is now one for-each-ref --merged, not one process per branch.
• Six branches are checked at a time.

Testing

• 4 new BranchOps.deleteRemotes tests: a normal batch, a branch pushed to since the fetch, one already gone on the remote, and one the server refuses.
• Full suite: 252 tests, 0 failures, from parsed result XML. detekt is clean.
• Not yet checked by hand in the running app.