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>
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 708f211..4fec1c1 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -1195,6 +1195,10 @@
   call feedkeys(":ka\<C-A>\<C-B>\"\<CR>", 'xt')
   call assert_equal("\"ka\<C-A>", @:)
 
+  " completion for :keepmarks command
+  call feedkeys(":kee edi\<C-A>\<C-B>\"\<CR>", 'xt')
+  call assert_equal("\"kee edit", @:)
+
   " completion for short version of the :s command
   call feedkeys(":sI \<C-A>\<C-B>\"\<CR>", 'xt')
   call assert_equal("\"sI \<C-A>", @:)
@@ -3901,7 +3905,7 @@
   let list = filter(getcompletion('', 'command'), 'exists(":" . v:val) == 0')
   " :++ and :-- are only valid in Vim9 Script context, so they can be ignored
   call assert_equal(['++', '--'], sort(list))
-  call assert_equal(1, exists(':k'))
+  call assert_equal(2, exists(':k'))
   call assert_equal(0, exists(':ke'))
   call assert_equal(1, exists(':kee'))
   call assert_equal(1, exists(':keep'))