Update the contents of an existing index object in memory by reading from the hard disk.

If force is true, this performs a "hard" read that discards in-memory changes and always reloads the on-disk index data. If there is no on-disk version, the index will be cleared.

If force is false, this does a "soft" read that reloads the index data from disk only if it has changed since the last time it was loaded. Purely in-memory index data will be untouched. Be aware: if there are changes on disk, unwritten in-memory changes are discarded.