rebase: add --insert-after and --insert-before options for `--rev…

…isions`

This allows for rebased commits to be moved before or after any given
commits.

The general steps of the approach taken are:
1. Compute the new parents and children of the rebased commits, based on
   the provided `--insert-after`/`--insert-before` options.
2. "Extract" the commits, by rebasing its descendants onto its parents
   (i.e. identical behavior to `jj rebase -r X -d X` before shifting the
   commit to its new destination)
3. Rebase the commits onto its new parents, then rebase the new children
   and its descendants accordingly using `MutRepo::transform_descendants`.

This means there are 2 separate rebasing steps: one for the descendants
of the rebased commits, and another to shift the rebased commits and its
new children.

This can lead to repeated rebasing of the same commits if the commit to
be rebased is either an ancestor or descendant of the new children.
However, I'm not sure if there's a way to do the rebasing in one step
using the current APIs: I'm not sure if there's a way to customize the
iteration order of commits in `transform_descendants` currently.

CHANGELOG.md

@@ -54,6 +54,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * `jj rebase` now accepts revsets resolving to multiple revisions with the
    `--revisions`/`-r` option.
 
+* `jj rebase -r` now accepts `--insert-after` and `--insert-before` options to
+  customize the location of the rebased revisions.
+
 ### Fixed bugs
 
 * Revsets now support `\`-escapes in string literal.