gitk: fix tabbed preferences construction when using tcl 8.4

gitk: fix tabbed preferences construction when using tcl 8.4

In 8.5 the incr command creates the target variable if it does not exist
but in 8.4 using incr on a non-existing variable raises an error. Ensure
we have created our counter variable when creating the tabbed dialog for
non-themed preferences.

Reported-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>