There are several existing commands that would benefit from an API
that makes it easier to rewrite a whole graph of commits while
transforming them in some way.
`jj squash` is one example. When squashing into an ancestor, that
command currently rewrites the ancestor, then rebases descendants, and
then rewrites the rewritten source commit. It would be better to
rewrite the source commit (and any descendants) only once.
Another example is the future `jj fix`. That command will want to
rewrite a graph while updating the trees. There's currently no good
API for that; you have to manually iterate over descendants and
rewrite them.
This patch adds a new `MutableRepo::transform_descendants()` method
that takes a callback which gets a `CommitRewriter` passed to it. The
callback can then decide to change the parents, the tree, etc. The
callback is also free to leave the commit in place or to abandon it.
I updated the regular `rebase_descendants()` to use the new function
in order to exercise it. I hope we can replace all of the
`rebase_descendant_*()` flavors later.
I added a `replace_parent()` method that was a bit useful for the test
case. It could easily be hard-coded in the test case instead, but I
think the method will be useful for `jj git sync` and similar in the
future.
