Fix for replaying amended parent commits during rebase

[skarim]

• Fixes gh stack rebase duplicates an amended downstack commit after pushing #250

Amending a lower branch and then running gh stack push could overwrite its
child's recorded base with a commit the child did not contain. The next
rebase would replay the old parent commit into the child, creating duplicate
history or unnecessary conflicts.

This preserves the last valid recorded base and only uses validated ancestor
commits as rebase --onto boundaries. Existing corrupted metadata can be
recovered from the parent reflog; when no safe boundary is available, the
command stops instead of guessing and rewriting history incorrectly.

Testing

• go vet ./...
• go test -race -count=1 ./...
• Real-Git coverage for amend → push → rebase
• Coverage for corrupted metadata recovery and safe no-reflog failure

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[Copilot]

Pull request overview

Prevents amended parent commits from being replayed during stacked rebases by preserving safe base metadata and recovering boundaries from reflogs.

Changes:

• Validates recorded rebase boundaries before updating or using them.
• Adds git merge-base --fork-point support.
• Adds unit and real-Git regression coverage for issue #250.

Show a summary per file

File	Description
internal/git/mock_ops.go	Adds fork-point mocking support.
internal/git/gitops.go	Implements the fork-point Git operation.
internal/git/git.go	Exposes the fork-point wrapper.
cmd/utils.go	Preserves and resolves safe rebase boundaries.
cmd/utils_test.go	Tests base preservation.
cmd/sync_test.go	Updates sync fork-point coverage.
cmd/rebase_test.go	Adds rebase unit and integration regressions.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 0
• Review effort level: Medium