Survey revisions/releases with partially loaded history

To be able to enable global deduplication of objects in the git loader without mediation via the extid table (and as long as swhids are 1:1 compatible with git identifiers), we need to make sure that the revisions and releases currently in the archive have been properly loaded, with their full history.

We need to survey this, to figure out:

how many history holes we have (revision/release objects with parents unknown in the archive)
how many history holes we can fix (by reloading a known origin containing the holey object and its parents)
whether the remaining history holes would prevent us from turning on global deduplication altogether, or not.

Migrated from T3656 (view on Phabricator)