Rebase

Rebase the current branch onto its base branch.

Step 1 — Determine the base branch

Read release-branches.md to get the list of branches. For each branch, compute the merge-base with HEAD and count the commits between them:

mb=$(git merge-base HEAD <branch>)
git rev-list --count "$mb"..HEAD

The base branch is whichever has the lowest commit count (fewest commits between the merge-base and HEAD). If counts are tied, prefer main.

Tell the user which base branch was detected before proceeding.