Run local git actions without blocking the main thread #732

GitX runs its local git actions synchronously on the main thread, so a slow command freezes the window until git finishes, or until PBTask gives up after 30 seconds.

Where it happens

• PBGitRepository runs checkout, merge, cherry-pick, reset, rebase, stash, branch and tag creation, ref deletion, and the worktree actions (lock, unlock, prune, add, remove, repair) through outputOfTaskWithArguments:, which ends in PBTask's blocking launchTask:.
• Only fetch, pull and push already run off the main thread.
• launchTask: terminates any command after 30 seconds and reports a timeout, so a long checkout or git worktree add in a large repository can be killed part way through.

What a fix involves

• An asynchronous path for local actions that delivers success or the error back on the main queue.
• A busy state while a command runs, so the same action cannot be started twice.
• Revisiting the 30-second limit for commands whose duration grows with the repository.
• Refreshing refs, the index and the worktree list once the command completes.

Testing

Callers assume the result is there when the call returns, so each action's flow (sheets, errors, refresh) needs checking end to end, not only through unit tests.

Raised by Copilot on #723 and #727; both PRs follow the existing synchronous pattern and leave this change to its own PR.