gitk: Show local uncommitted changes as a fake commit

If there are local changes in the repository, i.e., git-diff-index HEAD
produces some output, then this optionally displays an extra row in
the graph as a child of the HEAD commit (but with a red circle to
indicate that it's not a real commit).  There is a checkbox in the
preferences window to control whether gitk does this or not.

Clicking on the extra row shows the diffs between the working directory
and the HEAD (using git diff-index -p).  The right-click menu on the
extra row allows the user to generate a patch containing the local diffs,
or to display the diffs between the working directory and any commit.

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