libxdiff patches

I'm sending three patches that should apply cleanly on top of libxdiff 0.21; we use the first two with the copy of libxdiff/xdiff as built-in difff generator in git.

The first patch is to add an option to emit function names in the hunk headers, by Mark Wooding. I think this is a good addition to the upstream libxdiff itself.

The second one is to squelch compilation warnings on (seemingly) uninitialized variables, by Marco Roeland. You have already explained on the git list that these warnings are harmless (IOW the compiler is being stupid), but squelching these warnings would help spot other real problems.

The primary reason I am sending these to you is to keep the changes between your version and the copy git uses to the minimum, so we can keep up with your future updates. Right now, the differences are mostly that the copy git uses is stripped down (generalization like s_memallocator and s_mmblock are not used in our copy, for example), and these two patches are the only real additions we have.

The third patch, by Johannes Schindelin, teaches whitespaces to xdiff: -b (--ignore-space-change) and -w (--ignore-all-space). I haven't merged this into the mainline of git yet. If you are interested in adding this to upstream (I think it would be a good addition -- sanity checking is appreciated, though), and if you are going to do that slightly or majorly differently, I would prefer to use the change from upstream in order to avoid carrying git-only local changes in my tree.