git-gui: Refactor the add to commit state filters.

git-gui: Refactor the add to commit state filters.

The list of states which are valid for update-index were a little
too verbose and fed a few too many cases to the program.  We can
do better with less lines of code by using more pattern matching,
and since we already were globbing here there's little change in
runtime cost.

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