defaults: shortmess+=C #23907

Problem:
Completion messages such as "scanning tags" are noisy and generally not
useful on most systems. Most users probably aren't aware that this is
configurable.

Solution:
Set `shortmess+=C`.

src/nvim/options.lua

@@ -2233,7 +2233,7 @@ return {
       short_desc=N_("list of flags, reduce length of messages"),
       type='string', list='flags', scope={'global'},
       varname='p_shm',
-      defaults={if_true="filnxtToOF"},
+      defaults={if_true="filnxtToOCF"},
       cb='did_set_shortmess'
     },
     {