patch 9.1.0685: too many strlen() calls in usercmd.c

Problem:  too many strlen() calls in usercmd.c
Solution: refactor code to reduce the number or strlen() calls
          (John Marriott)

closes: #15516

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: John Marriott

src/testdir/test_usercommands.vim

@@ -133,6 +133,10 @@ function Test_cmdmods()
       \ 'silent verbose aboveleft belowright botright tab topleft vertical',
       \ g:mods)
 
+  kee keep keepm keepma keepmar keepmarks keepa keepalt keepj keepjumps
+      \ keepp keeppatterns MyCmd
+  call assert_equal('keepalt keepjumps keepmarks keeppatterns', g:mods)
+
   let g:mods = ''
   command! -nargs=* MyQCmd let g:mods .= '<q-mods> '