gitk: Show changes in index and changes in working directory separately

This makes gitk show up to two fake commits when there are local changes
in the repository; one to represent the state of the index and one to
represent the state of the working directory.  The commit representing
the working directory is colored red as before; the commit representing
the index state is colored magenta (as being between red and blue in
some sense).

Signed-off-by: Paul Mackerras <paulus@samba.org>