fsmonitor: convert shown khash to strset in do_handle_client

fsmonitor: convert shown khash to strset in do_handle_client

Replace the khash-based string set used for deduplicating pathnames
in do_handle_client() with a strset, which provides a cleaner
interface for the same purpose.

Since the paths are interned strings from the batch data, use
strdup_strings=0 to avoid unnecessary copies.

Suggested-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Paul Tarjan <github@paulisageek.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>