Add jujutsu support

Source at https://github.com/martinvonz/jj and hosted docs at
https://martinvonz.github.io/jj/latest/.

Author: msuozzo

README.md

@@ -8,7 +8,8 @@ removed lines in a file that is managed by a version control system (VCS)._
 ---
 
 - Supports **git**, **mercurial**, **darcs**, **bazaar**, **subversion**,
-  **cvs**, **rcs**, **fossil**, **accurev**, **perforce**, **tfs**, **yadm**.
+  **cvs**, **rcs**, **fossil**, **accurev**, **perforce**, **tfs**, **yadm**,
+  **jj**.
 - **Asynchronous** execution of VCS tools for Vim 8.0.902+ and Neovim.
 - **Preserves signs** from other plugins.
 - Handles **nested repositories** controlled by different VCS.
@@ -46,6 +47,7 @@ endif
 ```
 
 ## Configuration for async update
+
 ```vim
 " default updatetime 4000ms is not good for async update
 set updatetime=100

autoload/sy/repo.vim

@@ -179,6 +179,11 @@ function! s:check_diff_svn(exitval, diff) abort
   return a:exitval ? [0, []] : [1, a:diff]
 endfunction
 
+" s:check_diff_jj {{{1
+function! s:check_diff_jj(exitval, diff) abort
+  return a:exitval ? [0, []] : [1, a:diff]
+endfunction
+
 " s:check_diff_bzr {{{1
 function! s:check_diff_bzr(exitval, diff) abort
   return (a:exitval =~ '[012]') ? [1, a:diff] : [0, []]
@@ -629,7 +634,8 @@ let s:default_vcs_cmds = {
       \ 'rcs':      'rcsdiff -U0 %f 2>%n',
       \ 'accurev':  'accurev diff %f -- -U0',
       \ 'perforce': 'p4 info '. sy#util#shell_redirect('%n') . (has('win32') ? ' &&' : ' && env P4DIFF= P4COLORS=') .' p4 diff -du0 %f',
-      \ 'tfs':      'tf diff -version:W -noprompt -format:Unified %f'
+      \ 'tfs':      'tf diff -version:W -noprompt -format:Unified %f',
+      \ 'jj':       'jj diff --color=never --git --context=0 -r @ -- %f',
       \ }
 
 let s:default_vcs_cmds_diffmode = {
@@ -645,6 +651,7 @@ let s:default_vcs_cmds_diffmode = {
       \ 'accurev':  'accurev cat %f',
       \ 'perforce': 'p4 print %f',
       \ 'tfs':      'tf view -version:W -noprompt %f',
+      \ 'jj':       'jj file show -r @- -- %f',
       \ }
 
 if exists('g:signify_vcs_cmds')

doc/signify.txt

@@ -163,6 +163,7 @@ The key can be any from this list:
     accurev
     perforce
     tfs
+    jj
 
 Modifiers:~
 
@@ -201,7 +202,8 @@ Default:
       \ 'rcs':      'rcsdiff -U0 %f 2>%n',
       \ 'accurev':  'accurev diff %f -- -U0',
       \ 'perforce': 'p4 info '. sy#util#shell_redirect('%n') . (has('win32') ? ' &&' : ' && env P4DIFF= P4COLORS=') .' p4 diff -du0 %f',
-      \ 'tfs':      'tf diff -version:W -noprompt -format:Unified %f'
+      \ 'tfs':      'tf diff -version:W -noprompt -format:Unified %f',
+      \ 'jj':       'jj diff --color=never --context=0 -r @ -- %f',
       \ }
 <
 ------------------------------------------------------------------------------
@@ -219,6 +221,7 @@ Default:
       \ 'accurev':  'accurev cat %f',
       \ 'perforce': 'p4 print %f',
       \ 'tfs':      'tf view -version:W -noprompt %f',
+      \ 'jj':       'jj cat -- %f',
       \ }
 <
 The command to use for updating signs in a `modified` buffer, thus when the