Consider the following DAG:
C C' |\ /| A B D
with the goal of rebasing C to C' while switching the A parent to D.
Out of the box, rebase will fail here as it doesn't know which parent of
C to rewrite. With the new --parentmap option, this can be specified
explicitly. Use cases for this functionality are dealing with manual
rebases without obsolescence markers, e.g. git-style forced commits.