There isn’t a command to find the common ancestor per-se, but there is a
plumbing command, git-merge-base in Git for finding the best ancestor for use
in a three-way merge. Given a series of branches, git-merge-base attempts to
find the best ancestor to merge. When you provide it with two branches as
arguments, git-merge-base will naturally locate their common ancestor, if one
exists. This can be useful sometimes when you are trying to trace the common
ancestor between branches.
git-merge-base branchA branchB