SignifyDiff! will not open a new tab

This makes is behave like :SignifyFold.

Closes #271

autoload/sy/repo.vim

@@ -240,7 +240,7 @@ function! sy#repo#debug_detection()
 endfunction
 
 " Function: #diffmode {{{1
-function! sy#repo#diffmode() abort
+function! sy#repo#diffmode(do_tab) abort
   execute sy#util#return_if_no_changes()
 
   let vcs = b:sy.updated_by
@@ -252,7 +252,9 @@ function! sy#repo#diffmode() abort
   let cmd = s:expand_cmd(vcs, g:signify_vcs_cmds_diffmode)
   call sy#verbose('SignifyDiff: '. cmd, vcs)
   let ft = &filetype
-  tabedit %
+  if a:do_tab
+    tabedit %
+  endif
   diffthis
   let [cwd, chdir] = sy#util#chdir()
   try

doc/signify.txt

@@ -448,11 +448,13 @@ NOTE: Nothing will happen, if :SignifyRefresh is used from the |cmdline-window|.
 
 ------------------------------------------------------------------------------
                                                           *signify-:SignifyDiff*  >
-    :SignifyDiff
+    :SignifyDiff[!]
 <
 Open a new tab with two windows using |diff-mode| to show the differences
 between the current file and its version that was last checked in.
 
+With [!], no new tab will be opened.
+
 Also see |g:signify_vcs_cmds_diffmode|.
 
 ------------------------------------------------------------------------------

plugin/signify.vim

@@ -66,7 +66,7 @@ augroup END
 command! -nargs=0 -bar       SignifyList            call sy#debug#list_active_buffers()
 command! -nargs=0 -bar       SignifyDebug           call sy#repo#debug_detection()
 command! -nargs=0 -bar -bang SignifyFold            call sy#fold#dispatch(<bang>1)
-command! -nargs=0 -bar       SignifyDiff            call sy#repo#diffmode()
+command! -nargs=0 -bar -bang SignifyDiff            call sy#repo#diffmode(<bang>1)
 command! -nargs=0 -bar       SignifyRefresh         call sy#util#refresh_windows()
 command! -nargs=0 -bar       SignifyEnable          call sy#enable()
 command! -nargs=0 -bar       SignifyDisable         call sy#disable()