The `DescendantRebaser` keeps a map of branches from the source
commit, so it gets efficient lookup of branches to update when a
commit has been rebased. This map was not kept up to date as we
rebased. That could lead to branches getting left on hidden
intermediate commits. Specifically, if a commit with a branch was
rewritten by some command, and an ancestor of it was also rewritten,
then we'd only update the branch only the first step and not update it
again when rebasing onto the rewritten ancestor.
I noticed earlier today that branches get lost (stuck on a hidden
commit) when you move part of a change to an ancestor. This patch adds
tests for both of those cases, showing the bug. There's no special
logic for this case in the CLI crate, so we should be able to test it
in the library crate instead, but since I have already written the
tests, maybe we can keep them.
I quite often want to move the changes to a particular file from one
commit to another. We already support that using `jj move -i`, but
that can be annoying to run because we don't have a TUI for it
(#48). Let's make it possible to do `jj move --from X --to Y <path>`.
