Changes

Summary

1. removed obsolete self_update: always enabled, notably on lxbroy10 which is the only shared-home system (and still requires current isabelle_self);
2. avoid redundant meta data: exclude .hg_archival.txt;
3. clarified remote vs. local build_history: operate on hg_sync directory instead of repository;
4. proper operation on String, not Path;
5. clarified signature: cwd can be misleading --- changes meaning of target;