commit/diff search: `file:x y` returns changes of y in other files

Created by: camdencheek

Currently, in the new diff/commit search path, file:x y matches y in diffs that modify file x. However, I think the semantics here to be that file:x y matches y in hunks that match file:x. This will require merging file:x y into a single DiffMatches{Expr:"y", File:"x"}.