Add support for more trailers

regzbot relies fully on the Link: / Closes: tag to associate regression reports with fixes; commit-id and reported-by should suffice as well.

Start by reading docs/reference.md and locating the code that parses Link and Closes trailers to associate regression reports with fixes. Trace how commit-id and reported-by are currently handled, then verify that either trailer can establish the intended association without relying on Link or Closes.

Written by the indexing model from the issue text.