patch 9.1.0748: :keep* commmands are sometimes misidentified as :k

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: #15742

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    748,
 /**/
     747,
 /**/