hotfix for session sizes in tmux 2.6

See tmux 2.6 changes:

* All new sessions that are unattached (whether with -d or started with no
  terminal) are now created with size 80 x 24. Whether the status line is on or
  off does not affect the size of new sessions until they are attached.

This fix uses the -x and -y arguments (width and height) to server.
Right now we default to 800 width and 600 height.

Author: tony

libtmux/server.py

@@ -476,6 +476,10 @@ def new_session(self,
 
         if start_directory:
             tmux_args += ('-c', start_directory)
+
+        if has_gte_version('2.6'):
+            tmux_args += ('-x', 800, '-y', 600)
+
         proc = self.cmd(
             'new-session',
             *tmux_args