fast forward active branches where local is behind remote

[skarim]

Fast-forward stack branches that are behind their remote

In certain situations, when there was an upstream commit not present on local, gh stack rebase and gh stack sync would rebase on top of the stale local ref, effectively dropping the new commits.

Fix: Before the cascade rebase, both rebase and sync now detect stack branches where the local ref is strictly behind the remote tracking branch and fast-forward them. If the branch has diverged, it's left alone (the push step handles that). For sync, a branch fast-forward also triggers the cascade rebase even if trunk hasn't moved.

[Copilot]

Pull request overview

Adds support for fast-forwarding stack branches that are locally behind their remote counterpart, so sync/rebase operate on up-to-date branch tips before cascading rebases.

Changes:

• Introduce a shared fastForwardBranches helper to fast-forward active stack branches when local is strictly behind remote/<branch>.
• Run branch fast-forwarding during gh stack sync (and trigger cascade rebase when any branch moved) and during gh stack rebase.
• Update sync/rebase tests and mocks to model origin/<branch> SHAs and cover branch fast-forward scenarios.

Show a summary per file

File	Description
cmd/utils.go	Adds fastForwardBranches helper used by sync/rebase to fast-forward active branches behind the chosen remote.
cmd/sync.go	Calls fastForwardBranches and treats “any branch moved” as a reason to cascade rebase.
cmd/rebase.go	Calls fastForwardBranches before the cascade rebase begins.
cmd/sync_test.go	Adjusts mock SHA behavior and adds coverage for branch fast-forward-driven rebases/force pushes.
cmd/rebase_test.go	Adjusts mock SHA behavior and adds coverage for branch fast-forward behavior during rebase.

Copilot's findings

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• Files reviewed: 5/5 changed files
• Comments generated: 3