patch 9.2.0508: completion: cannot complete user cmd :K with 'ignorecase'

patch 9.2.0508: completion: cannot complete user cmd :K with 'ignorecase'

Problem:  completion: cannot complete user cmd :K with 'ignorecase'
          (rendcrx)
Solution: Skip the short-circuit when 'ignorecase' is set
          (Yasuhiro Matsumoto)

The set_cmd_index() short-circuit for the :k command treats ":k<X>" as
":k {X}" (mark argument), which makes ":kz<Tab>" never reach the
command-name expansion path. With 'ignorecase' the same prefix on other
letters (":gz<Tab>") completes a user command like :Gz, so the result is
inconsistent. Skip the short-circuit when 'ignorecase' is set; default
behaviour is preserved so the existing :k tests still pass.

fixes:  #20241
closes: #20275

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>