[pacman-dev] Add delta creation to repo-add.

[Nathan Jones]

For the first patch, I added the -n flag to gzip in order to prevent the
md5sum problem. The apply-delta script is not as efficient as it could
be. There is an extra gunzip+gzip step that could be avoided if we
create a delta on the .tar rather than the .tar.gz. Doing so would
complicate makepkg a bit, so I stuck with the extra step.

For the second patch, I created a second function, create_xdelta_latest,
which creates a delta for the latest package with a given package name.
I did this because otherwise a delta would be created for every package
given on the command line which is annoying for me because I use
*.pkg.tar.gz.

Problems:
* gzip upgrade could produce different md5sums
  - could have had the problem before if a zlib upgrade did the same thing
* all delta files that currently exist will not work (probably not a problem)