The GC now simply scans the directory and identifies index and binary
files to keep, based on last modification time alone. It no longer walks
the revision history, so missing revisions won't be a problem.
The "Keep N revisions" feature was removed, as it makes no sense. A user
might create 100 revisions a second, or none in a year. Revisions should
only be considered to allow MVCC, not be a 'history' of changes that one
might want to preserve. If so, GC must be disabled completely.