more uniform header syntax, in contrast to the former etc/abbrevs file-format (see 73939a9b70a3);

uniform graph restriction: build_graph is more sparse than imports_graph and may yield different results for exclude_session_groups / exclude_sessions (e.g. "isabelle build -a -X main");

updated comments;

updated to scala-2.12.4;

testing NewTestRegisterSave;

superseded by plain_identify;

tolerate odd negative times from old log files (before 1698e9ccef2d);

prefer main entry points of HOL;

tuned signature;

clarified lazy Completion within Outer_Syntax: measurable speedup of Sessions.deps;