git-gui: Add or unstage based on the specific icon used.

git-gui: Add or unstage based on the specific icon used.

Rather than relying on the file state and just inverting it, we should
look at which file icon the user clicked on.  If they clicked on the
one in the "Changes To Be Committed" list then they want to unstage
the file.  If they clicked on the icon in the "Changed But Not Updated"
list then they want to add the file to the commit.  This should be much
more reliable about capturing the user's intent then looking at the file
state.

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