gitk: Use git-diff-tree --cc for showing the diffs for merges

This replaces a lot of code that used the result from several 2-way
diffs to generate a combined diff for a merge.  Now we just use
git-diff-tree --cc and colorize the output a bit, which is a lot
simpler, and has the enormous advantage that if the diff doesn't
show quite what someone thinks it should show, I can deflect the
blame to someone else. :)

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