GUIs: redraw screen after sign update

Some GUIs (MacVim, gVim) wouldn't show placed signs after updating. Force a
redraw.

References #228.

autoload/sy/sign.vim

@@ -189,6 +189,12 @@ function! sy#sign#process_diff(sy, diff) abort
     execute 'sign unplace' a:sy.internal[line].id 'buffer='.a:sy.buffer
   endfor
 
+  if has('gui_running') && has('patch-7.4.1967')
+    " Some GUIs need this extra kick in the butt, when setting signs from the
+    " exit handler. :redraw would trigger a "hanging cursor bug" in MacVim.
+    call feedkeys("\<c-l>")
+  endif
+
   let a:sy.stats = [added, modified, deleted]
 endfunction