gitk: replace ${NS} with ttk

gitk uses ${NS} to select between unthemed and themed widgets, the
former are in the anonymous global namespace, while the themed ones are
in the ttk namespace.  However, gitk now always uses themed widgets, so
this indirection now serves no purpose. Let's switch to explicit use of
ttk:: via global search/replace, and make no other changes to keep this
commit more easily verifiable.

Signed-off-by: Mark Levedahl <[email protected]>

Author: mark987