Spawn tmux fg job in bg window when height is 0

When the requested height for a tmux split is 0 and that the spawned job
will be able to call back, spawn the foreground job in a new background
window, just as if it were a background job
Note that setting the height to 0 behaves gracefully even when no
callback mechanism is available since "split-window -l 0" creates a two
lines split (with tmux 2.5 to 2.7 at least)

Author: shym

autoload/dispatch/tmux.vim

@@ -49,7 +49,7 @@ function! dispatch#tmux#make(request) abort
 
   let title = shellescape(get(a:request, 'title', get(a:request, 'compiler', 'make')))
   let height = get(g:, 'dispatch_tmux_height', get(g:, 'dispatch_quickfix_height', 10))
-  if get(a:request, 'background', 0)
+  if get(a:request, 'background', 0) || (height <= 0 && dispatch#has_callback())
     let cmd = 'new-window -d -n '.title
   elseif has('gui_running') || empty($TMUX) || (!empty(''.session) && session !=# system('tmux display-message -p "#S"')[0:-2])
     let cmd = 'new-window -n '.title