git-gui: Clarify meaning of add tracked menu option

git-gui: Clarify meaning of add tracked menu option

Junio recently pointed out on the mailing list that our "Add Existing"
feature is a lot like `git add -u`, which is generally described as
"(Re)Add Tracked Files".  This came up during discussion of how to
translate "Add Existing" into Japanese, as the individual working on
the translation was not quite sure what the option meant and therefore
had some trouble selecting the best translation.

I'm changing the menu option to "Add Tracked Files To Commit" and the
button to "Add Tracked".  This should help new users to better understand
the actions behind those GUI widgets.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>