refactor(memline): distinguish mutating uses of ml_get_buf()

ml_get_buf() takes a third parameters to indicate whether the
caller wants to mutate the memline data in place. However
the vast majority of the call sites is using this function
just to specify a buffer but without any mutation. This makes
it harder to grep for the places which actually perform mutation.

Solution: Remove the bool param from ml_get_buf(). it now works
like ml_get() except for a non-current buffer. Add a new
ml_get_buf_mut() function for the mutating use-case, which can
be grepped along with the other ml_replace() etc functions which
can modify the memline.

src/nvim/spellsuggest.c

@@ -2816,7 +2816,7 @@ static void add_sound_suggest(suginfo_T *su, char *goodword, int score, langp_T
   }
 
   // Go over the list of good words that produce this soundfold word
-  char *nrline = ml_get_buf(slang->sl_sugbuf, (linenr_T)sfwordnr + 1, false);
+  char *nrline = ml_get_buf(slang->sl_sugbuf, (linenr_T)sfwordnr + 1);
   int orgnr = 0;
   while (*nrline != NUL) {
     // The wordnr was stored in a minimal nr of bytes as an offset to the