rerere: plug conflict ID leaks

rerere: plug conflict ID leaks

The merge_rr string list stores the conflict ID (a hexadecimal
string that is used to index into $GIT_DIR/rr-cache) in the .util
field of its elements, and when do_plain_rerere() resolves a
conflict, the field is cleared.  Also, when rerere_forget()
recomputes the conflict ID to updates the preimage file, the
conflict ID for the path is updated.

We forgot to free the existing conflict ID when we did these two
operations.

Signed-off-by: Junio C Hamano <gitster@pobox.com>