Add jujutsu support

[msuozzo]

patch originally authored by @algmyr

while the default diff does work, the 'working-copy-as-a-commit' semantics of jj mean that it may be preferable to customize the diff range to include the last non-anonymous commit. Unfortunately, there isn't currently an easy way to express this but the following should approximate it:

let g:signify_vcs_cmds.jj = 'jj diff --color=never --git --context=0 --from=''parents(heads(::@ & description(glob:"?*"))&mutable()) | ~(heads(::@ & description(glob:"?*"))&mutable())&heads(::@ & description(glob:"?*"))'' -- %f'

[msuozzo]

@jamessan seems the most recent reviewer

[eyvind]

LGTM, minor comment inline.

[eyvind]

jj should also be added to the list in the intro section of this document.

[entropitor]

Can / should we (optionally) use --ignore-working-copy? I'm getting a lot of divergent operations without this

[toxadx]

I agree, we should definitely use the --ignore-working-copy flag. Without it, jj creates a snapshot of the working directory when invoked from within vim, which leads to a lot of divergent operations.

[algmyr]

You want --ignore_working_copy for this yeah.

FWIW, since the time I authored the inital patch I ended up writing my own neovim plugin that takes inspiration from vim-signify (because I love the idea of having a vcs agnostic core).