vim-patch:9.0.1064: code for making 'shortmess' temporarily empty is repeated

Problem:    Code for making 'shortmess' temporarily empty is repeated.
Solution:   Add functions for making 'shortmess' empty and restoring it.
            (Christian Brabandt, closes vim/vim#11709)

9aee8ec400

Co-authored-by: Christian Brabandt <cb@256bit.org>

src/nvim/option_defs.h

@@ -260,6 +260,7 @@ enum {
   SHM_RECORDING      = 'q',  ///< Short recording message.
   SHM_FILEINFO       = 'F',  ///< No file info messages.
   SHM_SEARCHCOUNT    = 'S',  ///< Search stats: '[1/10]'
+  SHM_LEN            = 30,   ///< Max length of all flags together plus a NUL character.
 };
 /// Represented by 'a' flag.
 #define SHM_ALL_ABBREVIATIONS ((char[]) { \