Problem: The :keep{alt,jumps,marks,patterns} commmands are sometimes
misidentified as :k.
Solution: Make sure one_letter_cmd() only returns true for :k and not
other :keep* commands (Doug Kearns).
This currently manifests as missing completion for :keep* commands and
incorrect results from fullcommand().
E.g., fullcommand("keepmarks") returns "k" rather than "keepmarks".
The correct command, however, is executed as command modifiers are
handled specially in do_one_cmd() rather than using find_ex_command().
Fix exists(':k') so that it returns 2 for a full match.
closes: vim/vim#15742ea84202372
Cherry-pick Test_ex_command_completion() from patch 9.1.0624.
Co-authored-by: Doug Kearns <dougkearns@gmail.com>