Allow choice between #where or #from.
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.