Changes

Summary

1. proper remote_mac according to Admin/Release/CHECKLIST;
2. more Java heap, e.g. relevant for hg.graph on Isabelle repository;
3. more robust check -- do not rely on return code;
4. more compact ML_Statistics, to make build_status work with less than 2GB heap;
5. retain latest ml_stats (amending e76c6cb0d461);
6. accomodate smaller build machine;
7. clarified signature;
8. clarified self_update: imitate visible directory state on remote side;
9. more operations;
10. self_update implies push_isabelle_home (see also 4c253e84ae62);
11. uniform setup_repository (pull/clone without update); tuned signature;
12. clarified signature: facilitate interactive experimentation;
13. clarified tasks;
14. tuned signature;