git-gui: Correct DD file state to be only D_.

git-gui: Correct DD file state to be only D_.

Apparently my earlier suspicion that the file state DD was a bug was
correct.  A file which has been deleted from the working directory and
from the index will always get the state of D_ during a rescan.  Thus
the only valid state for this to have is D_.  We should always use only
D_ internally during our state changes.

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