pack: Prevent duplicate files in tar archives.

pack: Prevent duplicate files in tar archives.

Tar translate duplicate files in the archive into hard links.  These can cause
problems, as not every tool support them; for example dpkg doesn't.

* gnu/system/file-systems.scm (reduce-directories): New procedure.
(file-prefix?): Lift the restriction on file prefix.  The procedure can be
useful for comparing relative file names.  Adjust doc.
(file-name-depth): New procedure, extracted from ...
(btrfs-store-subvolume-file-name): ... here.
* guix/scripts/pack.scm (self-contained-tarball/builder): Use
reduce-directories.
* tests/file-systems.scm ("reduce-directories"): New test.