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_exists.vim b/src/testdir/test_exists.vim
index 9cdb3fb..404b15c 100644
--- a/src/testdir/test_exists.vim
+++ b/src/testdir/test_exists.vim
@@ -113,6 +113,13 @@
   " Internal command with a count
   call assert_equal(0, exists(':3buffer'))
 
+  " Valid internal command (full match)
+  call assert_equal(2, exists(':k'))
+  " Non-existing internal command (':k' with arg 'e')
+  call assert_equal(0, exists(':ke'))
+  " Valid internal command (partial match)
+  call assert_equal(1, exists(':kee'))
+
   " User defined command (full match)
   command! MyCmd :echo 'My command'
   call assert_equal(2, exists(':MyCmd'))