This adds a file predicate to filter to only files that contain content
matching the given pattern.
In textsearch cases, it's usually simpler to just use an "and" operator.
For example, file:contains.content(q1) q2 is equivalent to q1 and q1.
However, for non-file results, that's not possible. Consider "find diffs
that modify files that contain q1". With this change, that query can be
written as type:diff file:contains.content(q1) whereas that would
previously be unrepresentible in our query language.
