Both Mercurial and Git (xdiff) have a special case for empty hunks.
https://repo.mercurial-scm.org/hg/rev/2b1ec74c961f
I also changed the internal line numbers to start from 0 so we wouldn't have
to think about whether "N - 1" would underflow.
Fixes#5049
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
