This in itself is not an improvement, because hashes are unique (they are shared
between real commits and rebase todos, but there are so few of those that it
doesn't matter). However, it becomes an improvement once we also store parent
hashes in the same pool; but the real motivation for this change is to also
reuse the hash pointers in Pipe objects later in the branch. This will be a big
win because in a merge-heavy git repo there are many more Pipe instances than
commits.
