This patch makes `jj squash` us the helper I just extracted from `jj
move`. I had a to add a few small features to it for that.
The `test_squash_command.rs` test changed in a few cases where we do a
partial squash. After this patch, we include the rebased child in the
count of rebased descendants. That seems reasonable and consistent
with partial squash/move further than 1 generation.