feat: handle squash-merged parent PRs with --onto rebase

[boneskull]

When a parent PR is squash-merged into trunk, the original commits
become unreachable. A simple 'git rebase trunk' would try to replay
those commits, causing false conflicts.

This change:

• Adds RebaseOnto method (git rebase --onto) to git package
• Adds CommitExists method for SHA validation
• Stores fork points in git config (branch..stackForkPoint)
• Records fork point when creating/adopting branches
• Uses --onto rebase when fork point differs from current merge-base
• Updates fork point after successful cascade
• Cleans up fork point when orphaning branches
• Sync now calculates fork points before deleting merged branches

The fork point tracks where a branch originally diverged from its
parent, enabling precise commit replay even after parent modification.

Co-Authored-By: Claude Opus 4.5 noreply@anthropic.com

[boneskull]

Stack

• main
  • push-updates
  • feat: handle squash-merged parent PRs with --onto rebase #10 - branch: squash-merge-handling (this PR)

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Managed by gh-stack

[Copilot]

Pull request overview

Adds fork-point tracking and git rebase --onto support so stacked branches can be rebased correctly even when a parent PR was squash-merged (making the parent’s original commits unreachable).

Changes:

• Introduces CommitExists and RebaseOnto to the internal git wrapper.
• Persists per-branch fork points in .git/config and wires this into create/adopt/cascade/sync/orphan flows.
• Adds unit tests covering fork point config and the new git operations.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
internal/git/git.go	Adds CommitExists and RebaseOnto git helpers used by sync/cascade logic.
internal/git/git_test.go	Adds tests for CommitExists and RebaseOnto.
internal/config/config.go	Adds fork point storage/retrieval/removal in git config.
internal/config/config_test.go	Adds tests for fork point config behavior.
cmd/create.go	Records fork point when creating a stacked branch.
cmd/create_test.go	Adds coverage asserting fork point is stored during create flows.
cmd/adopt.go	Records fork point when adopting an existing branch into the stack.
cmd/adopt_test.go	Adds coverage asserting fork point is stored during adopt flows.
cmd/cascade.go	Uses fork point to decide when to rebase --onto and updates fork point after successful cascade.
cmd/sync.go	Retargets children of merged branches, collects fork points before deletion, and uses --onto where applicable.
cmd/orphan.go	Cleans up fork point metadata when orphaning branches (and descendants).
cmd/orphan_test.go	Adds coverage asserting fork point is removed when orphaning.

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