ContentPartitionIndexer: Do not index the same content multiple times at once.
self._index_contents was called multiple times in a loop with the same arguments, except for the set of hashes to exclude.
It means that, if there were N pages of hashes to exclude, each content was indexed N times; and the N-1 first iterations didn't even exclude all the hashes they had to exclude.
Resolves SWH-INDEXER-93 and SWH-INDEXER-7R
(w/ @ardumont)
Migrated from D4982 (view on Phabricator)