cli: add "absorb" machinery and command

The destination commits are selected based on annotation, which I think is
basically the same as "hg absorb" (except for handling of consecutive hunks.)
However, we don't compute a full interleaved delta right now, and the hunks are
merged in the same way as "jj squash". This means absorbed hunks might produce
conflicts if no context lines exist. Still I think this is more intuitive than
selecting destination commits based on patch commutativity.

I've left inline comments to the tests where behavior is different from "hg
absorb", but these aren't exhaustively checked.

Closes #170

CHANGELOG.md

@@ -20,6 +20,8 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 * The `jj desc` and `jj st` aliases are now hidden to not interfere with shell
   completion. They remain available.
 
+* New command `jj absorb` that moves changes to stack of mutable revisions.
+
 * New command `jj util exec` that can be used for arbitrary aliases.
 
 * A preview of improved shell completions was added. Please refer to the