git-gui: Auto-update any A? or M? files during rescan.

If the user has partial includes disabled then it doesn't matter what
state the working directory is in; if the file has been included in
the next commit its index state is A or M and we should immediately
run update-index on the working directory file to bring the index in
sync with the working directory.

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