provide hugo-0.88.1 for darwin and linux;

removed obsolete self_update: always enabled, notably on lxbroy10 which is the only shared-home system (and still requires current isabelle_self);

avoid redundant meta data: exclude .hg_archival.txt;

clarified remote vs. local build_history: operate on hg_sync directory instead of repository;

proper operation on String, not Path;

clarified signature: cwd can be misleading --- changes meaning of target;

merged

more meta data; clarified signature: more explicit types;

clarified signature: more operations;

tuned messages;