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]

Shouldn't this be getting the file contents from @- (ie, jj cat -r @- -- %f)?

The jj diff in default_vcs_commands shows the diff between @- and @, and practically speaking, @- is jj's HEAD analogue.

It's more complicated if @ is a merge, since that would involve synthesising a merged version of the parents' files like jj diff does. I don't have a good solution for that, unfortunately.

[algmyr]

That sounds like it might be what we want yeah. Tbh I'm still not sure exactly sure what signify uses the different commands for.

Some quick testing: With the current setup it seems to default to show the diff compared to @-,but with edits to the buffer it shows the diff compared to @. Tbh, I don't hate that behavior but I suspect it's not what is expected compared to the other vcs implementations here. With -r @- it behaves more like you would expect. So +1 to fixing that

[algmyr]

Also, jj cat is deprecated, jj file show would be the new spelling.