add -p: only bind search key if there's more than one hunk

If there is only a single hunk then disable searching as there is
nothing to search for. Also print a specific error message if the user
tries to search with '/' when there's only a single hunk rather than
just listing the key bindings.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>