git-gui: call do_quit before destroying the main window

git-gui: call do_quit before destroying the main window

If the toplevel window for the window being destroyed is the main window
(aka "."), then simply destroying it means the cleanup tasks are not
executed (like saving the commit message buffer, saving window state,
etc.)

All this is handled by do_quit.  Call it instead of directly
destroying the main window. For other toplevel windows, the old
behavior remains.

Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>