git-merge-base

Find common ancestor

$ git merge-base [branch1] [branch2]

$ git merge-base --fork-point [branch]

$ git merge-base --is-ancestor [commit1] [commit2]

$ git merge-base --octopus [branch1] [branch2] [branch3]

git merge-base [options] commit...

git merge-base finds the best common ancestor(s) for a merge. The result is the most recent commit reachable from all specified commits, serving as the base for three-way merge operations.
The `--fork-point` option finds where a branch diverged from an upstream branch, accounting for history rewrites. The `--is-ancestor` option checks if one commit is an ancestor of another, which is useful in scripts for determining commit relationships. For octopus merges (merging more than two branches), the `--octopus` option computes a common base for multiple commits simultaneously.

--fork-point

Find fork point.

Check ancestry.

Find octopus merge base.

Output all merge bases.

git-merge(1), git-rebase(1)