revision metadata indexer: Fix multiple errors

This diffs fixes quite some initialization issues and other errors (variable initialization, logging format exception) in the revision metadata indexer.

Note for later. We need to simplify the initialization of the indexers. This has become far too tedious in its current state.

For this, I think we could:

• remove the prepare() indirection
• and use bare init (which is more pythonic and expected)
• to override the configuration in tests, override the parse_config_file (coming from the SWHConfig instead of overriding prepare)

Test Plan

tox

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Migrated from D712 (view on Phabricator)