Bug#648632: Documemtation on repack and compression

[Osamu Aoki]

control: retitle -1 Update document for repack and compress

The repack does not always repack with gzip anymore.  So documentation
needs updated accordingly.  Some of it has been done in the git
repository.

As I read this and think about the current situation, for the non-1.0
format, it may be better to use xz for repack as the default mode.
(This is todo)

It is known to reduce package size for font packages by going from gz to
xz and there are many packages like that.

As I understand, automatic repackage is done only for$mime= zip files
now with current mk-origtargz. (James' 2013 comment is outdated now.)

Some comments in the source code needs to be updated too.

Osamu