Changes

Changes from Mercurial (hg http://isabelle.in.tum.de/repos/isabelle/ default)

Summary

1. merged
2. more exports from rendering; suppress empty exports;
3. clarified signature;
4. clarified options: batch-build has pide_reports disabled by default (requires significant resources);
5. tuned;
6. proper return code for more errors (amending d892f6d66402);
7. more exports, as in "isabelle dump";
8. tuned;
9. clarified signature;
10. clarified signature: initial markup is_empty, not init_markup;
11. clarified signature: prefer high-level Snapshot over low-level Command.State;
12. tuned;
13. more strict;
14. clarified signature;
15. tuned;
16. clarified signature;
17. clarified signature --- avoid repeated open_database on server;