git-gui: Correct toggling of added/untracked status for new files.

git-gui: Correct toggling of added/untracked status for new files.

New files also lack index data from diff-files therefore we cannot use
their diff-files index data when we update-index.  Instead we can use
the fact that Git has them hardcoded as "0 0{40}" and do the same thing
ourselves.  This way you can toggle an untracked file into added status
and back out to untracked.

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