Support occur command operating on the region from Isearch.

Support occur command operating on the region from Isearch.

* lisp/isearch.el (isearch-occur): Use region-bounds as region arg of occur.
(isearch-query-replace): Use use-region-p.

* lisp/replace.el (occur--region-start-line): Rename from
occur--matches-threshold.
(occur): Use complete lines when region is active for line-oriented occur.
(occur-engine): Count lines either from occur--region-start-line or 1.