Reading an mtree file is supposed to provide

Reading an mtree file is supposed to provide
access to the file data (if the file exists on
disk).  This was broken for the first regular
file; fix it and add a test so it won't break again.

In particular, this fixes the following idiom for creating
a tar archive in which every file is owned by root:

tar cf - --format=mtree . 	\
    | sed -e 's/uname=[a-z]*/uname=root/' -e 's/uid=[0-9]*/uid=0/' \
    | tar cf - @-