git-gui: Make consecutive icon clicks toggle included status of a file.

git-gui: Make consecutive icon clicks toggle included status of a file.

If the user clicks on the icon associated with a file we now flip to the
inverse status.  Partially included files first fully include, then fully
uninclude, as we don't keep track of intermediate partial inclusions.

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