fsmonitor: refactor initialization of fsmonitor_last_update token

fsmonitor: refactor initialization of fsmonitor_last_update token

Isolate and document initialization of `istate->fsmonitor_last_update`.
This field should contain a fsmonitor-specific opaque token, but we
need to initialize it before we can actually talk to a fsmonitor process,
so we create a generic default value.

Signed-off-by: Jeff Hostetler <redacted>
Reviewed-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>