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 <firstname.lastname@example.org>