patch 9.1.1618: completion: incorrect selected index returned from complete_info()

patch 9.1.1618: completion: incorrect selected index returned from complete_info()

Problem:  completion: incorrect selected index returned from
          complete_info()
Solution: Return the index into "items" and restore the previous
          behaviour (Robert Muir).

complete_info() returned an incorrect selected index after
0ac1eb3555445f4c458c06cef7c411de1c8d1020 (Patch v9.1.1311). Effectively
it became an index into "matches" instead of "items". Return the index
into "items" by default to restore the previous behavior, unless
"matches" was requested.

closes: #17952

Co-authored-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Robert Muir <rmuir@apache.org>
Signed-off-by: Christian Brabandt <cb@256bit.org>