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/normal.c

@@ -1638,7 +1638,7 @@ size_t find_ident_at_pos(win_T *wp, linenr_T lnum, colnr_T startcol, char **text
 
   // if i == 0: try to find an identifier
   // if i == 1: try to find any non-white text
-  char *ptr = ml_get_buf(wp->w_buffer, lnum, false);
+  char *ptr = ml_get_buf(wp->w_buffer, lnum);
   for (i = (find_type & FIND_IDENT) ? 0 : 1; i < 2; i++) {
     // 1. skip to start of identifier/text
     col = startcol;