gitk: Fix the Makefile to cope with systems lacking msgfmt

gitk: Fix the Makefile to cope with systems lacking msgfmt

The po2msg.sh script and the .gitignore in the po directory have been
shamelessly copied from the current git-gui.  This enables the top
level "make NO_MSGFMT" to work consistently for git across the git-gui
and gitk sub-projects.

This is the same effective patch that has previously been posted as a
git.git patch which more succinctly described the copying of
po/.gitignore and po/po2msg.sh from git-gui.

Signed-off-by: Charles Bailey <charles@hashpling.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>