git-gui: Refactor file state representations.

git-gui: Refactor file state representations.

It just felt wrong to me that I was using _ as part of the mode argument
to display_file to mean "don't care/use existing" and * as part of
the mode argument to mean "force to _".

So instead use ? to mean "don't care/use existing" and _ to mean
"force to _".  The code is a lot clearer this way and hopefully it
won't drive another developer insane, as it did me.

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