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).
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
removing b
created new head
% should abort
abort: crosses branches (use 'hg merge' to merge or use 'hg update -C' to discard changes)
% should succeed
1 files updated, 0 files merged, 0 files removed, 0 files unresolved