Display of merges in gitk

I have reworked the way gitk displays merges. Previously it would display all the diffs between the child and each parent. That displayed a lot of unnecessary stuff; for example, for the famous octopus (pentapus?) merge, each diff was shown 4 times. It also shows diffs when the merge is perfectly straightforward, i.e. when any given file has been modified in at most one of the parents, and the child's version is the same as the parent's.

Now gitk will only list a file as having a difference in a merge if one or more of the parents has a version of the file that is different both from the child and from the common ancestor of the parents (if a common ancestor exists). If there is no common ancestor, then a file is listed if it is different in the child from all of the parents.

I now also display the diffs for a file in one unified difference listing. Lines are color-coded according to which parent they come from, and are in bold with a + at the start of the line if they ended up in the merge result (the child), or in normal font with a - at the start of the line if they didn't. In the usual case of two parents, lines from the first parent are in red and lines from the second parent are in blue. Lines in the result that don't correspond to either parent are in bold black.

Linus, could you do a pull from the usual place to pick this up? (rsync://rsync.kernel.org/pub/scm/gitk/gitk.git). I also included a patch from Junio.

Paul.