Immediately using #from here requires a lot of changes in
other finders (e.g. IssuableFinder, TodosFinder). In all places where we
use #merge, this goes completely the wrong way when passed in a relation
that was built with `#from(...)`: The original query's FROM part gets
completely replaced.

This avoids changing all other places and focuses on improving
SnippetFinder with the downside of two (small) codepaths to do the same
thing.