As the updates test case shows, when rebasing forward, we missed
commits that fork off from the section between the source and the
destination.
As part of the fix, I also restructured the code a bit to prepare for
support for rebasing descendants of multiple rewritten commits.
When I added the function for rebasing descendants, I forgot to call
the existing `rebase()` function and instead simply created a new
commit with the new parents but the old contents.
This should be useful in lots of places. For example, `jj rebase -r`
currently rebases all descendants, because that's what the auto-evolve
feature does. I think it would be nice to instead copy from
Mercurial's `-s` flag for also rebasing descendants. Then `jj rebase
-r` can be made to pull a commit out of a stack, rebasing descendants
onto the rebased commit's parents. I also intend to use this
functionality for rebasing descendants when remote branches have been
rewritten.
