tar-tree: Use the prefix field of a tar header

tar-tree: Use the prefix field of a tar header

... to store parts of the path, if possible.  This allows us to avoid
writing extended headers in certain cases (long pathes can only be
split at '/' chars).

Also adds a file to the test repo with a 100 chars long directory name.
Even old versions of tar that don't understand POSIX extended headers
should be able to handle this testcase.

Btw.: The longest path in the kernel tree currently has 70 chars.
Together with a 30 chars long prefix this would already cross the
field limit of 100 chars.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>