archival: remove prefix argument from archivers
When the archivers work on the full we can reuse the same archiver
with different prefixes (for different subrepositories).
#!/bin/sh
hg init
python -c 'print "confuse str.splitlines\nembedded\rnewline"' > a
hg ci -Ama -d '1 0'
echo clean diff >> a
hg ci -mb -d '2 0'
hg diff -r0 -r1