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