gitk: Ignore gitk-wish buildproduct

gitk: Ignore gitk-wish buildproduct

gitk, when bound into the git.git project tree, used to live at the
root level, but in 62ba514 (Move gitk to its own subdirectory,
2007-11-17) it was moved to a subdirectory.  The code used to track
changes to TCLTK_PATH (which should cause gitk to be rebuilt to
point at the new interpreter) was left in the main Makefile instead
of being moved to the new Makefile that was created for the gitk
project.

Also add .gitignore file to list build artifacts for the gitk
project.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>