git-gui: Only permit selection in one list at a time.

git-gui: Only permit selection in one list at a time.

Now that our lists represent more defined states it no longer makes any
sense to permit a user to make selections from both lists at once, as
the each available operation acts only on files whose status corresponds
to only one of the lists.

Signed-off-by: Shawn O. Pearce <redacted>