Fix finishing :Gcommit on :wqa

This was added to compensate for the implicit autowrite we lost when
switching from :! to system(), but it seems calling in the
VimLeavePre/BufLeave event prevents the commit from happening on exit.
I don't fully understand it but it's easy to prevent.

Closes https://github.com/tpope/vim-fugitive/issues/1285

autoload/fugitive.vim

@@ -2983,7 +2983,7 @@ function! s:CommitCommand(line1, line2, range, count, bang, mods, reg, arg, args
         let exec_error = v:shell_error
         call delete(errorfile)
       else
-        if &autowrite
+        if &autowrite && !a:0
           wall
         endif
         let [error_string, exec_error] = s:TempCmd(outfile, command)