git-gui: Teach the gui how to uninclude a file.

git-gui: Teach the gui how to uninclude a file.

Sometimes the user may want to keep their working directory file to be
the same content but they don't want it to be part of the current commit
anymore.  In this case we need to undo any changes made to the index
for that file (by reloading the info from HEAD or removing the file
from the index) but leave the working directory alone.

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