Function decideManagedRefresh

Whether an existing copy of a managed file may be replaced by the shipped one.

• older marker: refreshed only if it is byte-for-byte (line endings aside) a version we shipped with a marker; an edited copy keeps its marker line, so the marker alone proves nothing;
• current or newer marker: nothing to do;
• no marker: refreshed only if it is a version we shipped before markers existed. Anything else is the user's and is left alone ('edited').