The introduction of advanced search syntax allowed queries like `-foo`. These
break our repository queries in two distinct ways:

* Blobs would be returned for commit queries, and vice-versa
* Highlighting was assumed to be present for all queries

The first is fixed by adding explicit type terms to the queries:

    { term: { 'commit.type' => 'commit' } }
    { term: { 'blob.type' => 'blob' } }

(The `_type` field is, of course, `repository` for both document kinds).

A query matched by a negative search will not have any highlighting results,
so we also need to make processing that conditional.

Finally, switch commit search to `default_operator: :and` so it behaves more
like the rest of the application.

Switching to `default_operator: :and` should be enough to keep this from
happening.