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)