When the trunk branch (e.g. main) doesn't exist locally — only the
remote tracking ref (origin/main) exists — `fastForwardTrunk` called
`git rev-parse main origin/main` which failed, emitting:
⚠ Could not compare trunk main with remote — skipping trunk update
This also caused `stackNeedsRebase` to always return true (since
`IsAncestor("main", ...)` errors out), forcing an unnecessary rebase
and force-push on every sync.
Add a `BranchExists` check at the top of `fastForwardTrunk`. If the
local trunk doesn't exist, return silently — there's nothing to
fast-forward, and the remote tracking ref is sufficient for rebasing
via git's DWIM resolution.
Sameen Karim
8c2d9e3da6