Use rebase to align checkpoint branch with remote

[pjbgf]

Replace merge commits with rebase, making the checkpoint history linear.

Manually testing on fresh single-branch clones without the metadata branch before the changes:

After the changes:

---

Note

Medium Risk
Changes the non-fast-forward recovery path for pushing checkpoint metadata, replacing merge commits with a rebase/cherry-pick flow that uses git merge-base/git rev-list; mistakes here could lose or resurrect checkpoint shards during sync.

Overview
Checkpoint push conflict recovery now prefers a linear history. When pushing entire/checkpoints/v1 fails with non-fast-forward, the CLI now fetches and rebases local checkpoint commits onto the remote tip (with fast-forward shortcuts) instead of creating merge commits.

This adds merge-base/commit-range computation via git merge-base and git rev-list to identify and replay only local-only commits onto the remote, aiming to avoid replaying older ancestors (e.g., previously merged history) and keeping remote deletions intact.

Tests were updated/expanded: the integration concurrent-push test now asserts a single-parent tip, and new unit tests cover diverged branches, local-behind fast-forward, and the “merge-base on second parent” scenario to ensure no unintended ancestor replay.

^{Reviewed by Cursor Bugbot for commit 6e192c6. Configure here.}

[Copilot]

Pull request overview

This PR changes the checkpoint-metadata sync behavior during push retries to prefer a linear history by rebasing local metadata commits onto the remote tip, rather than creating merge commits.

Changes:

• Replace the previous fetch+merge recovery with a fetch+rebase (cherry-pick) approach for the metadata branch when a push is rejected.
• Add helpers to compute merge-base / collect commit ranges and apply them as linear commits.
• Add unit tests for diverged/fast-forward/legacy-merge-history scenarios and update an integration test to assert linear history.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
cmd/entire/cli/strategy/push_common.go	Switch non-fast-forward recovery from merge to rebase-like cherry-picking; add merge-base/rev-list helpers.
cmd/entire/cli/strategy/push_common_test.go	Add unit tests validating linearization + preservation of checkpoint data under several divergence scenarios.
cmd/entire/cli/integration_test/remote_operations_test.go	Update concurrent push integration test to expect rebased (non-merge) metadata history.

The merge-base changed after approval.