Fix duplicated prefix path in imenu entries
Fix duplicated prefix path in imenu entries

The entry already has the prefix path incorporated, so adding the
prefix again results in duplicate prefixes (in fact, n^2 module path
components!).

ocaml/merlin,ocaml/merlin,ocaml/merlin,rgrinberg/merlin,rgrinberg/merlin,rgrinberg/merlin,ocaml/merlin,rgrinberg/merlin