* Add script to backport a merged PR to a release branch
.github/backport.sh cherry-picks a merged PR's commit onto a release
branch and opens a backport PR mirroring the shape of #1842.
- Infers the target release from the PR's milestone (v1.12.0 -> release-1.11);
an explicit target-version argument overrides it.
- Requires a clean working tree (untracked files allowed) before starting.
- Prompts with the exact push and gh pr create commands before running them.
- Sets the new PR's milestone to the highest open v<target>.* milestone.
- Removes the needs-backport label from the original PR once the backport opens.
- Supports --continue to resume after resolving cherry-pick conflicts.
* cleanup
* just fetch all of origin
We need to ensure we have MERGE_SHA available too
