[PATCH] Initial Japanese gitk translation

Hello everyone.

My name is Luis Fors. I'm not a great Japanese speaker, and I have almost 
zero knowledge of tcl and getttext, but all of my coworkers are Japanese, 
and I couldn't just wait around for a translation to appear.

This is my first time contributing, and I have no idea what I'm doing, so be 
please be gentle.

This is a patch against gitk.git 5fdcbb13904e6...

Feel free to bang this into whatever shape is necessary to include this in 
the next release.

Thank you.