SignifyDiff: support Perforce

References #253

autoload/sy/repo.vim

@@ -463,6 +463,7 @@ let s:default_vcs_cmds_diffmode = {
       \ 'bzr':      'bzr cat %f',
       \ 'darcs':    'darcs show contents -- %f',
       \ 'cvs':      'cvs up -p -- %f 2>%n',
+      \ 'perforce': 'p4 print %f',
       \ }
 
 if exists('g:signify_vcs_cmds')