HgLoaderFromDisk: Don't query revision_get with release ids
This is a minor performance optimization, removing items from the call to revision_get when we know their result will be None.
Motivation: A future commit (swh-loader-core#3140 (closed)) will refactor this function, and dealing only with revision ids makes it simpler.
Migrated from D5369 (view on Phabricator)