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 - @-
