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/change.c

@@ -609,7 +609,7 @@ bool file_ff_differs(buf_T *buf, bool ignore_empty)
   if (ignore_empty
       && (buf->b_flags & BF_NEW)
       && buf->b_ml.ml_line_count == 1
-      && *ml_get_buf(buf, (linenr_T)1, false) == NUL) {
+      && *ml_get_buf(buf, (linenr_T)1) == NUL) {
     return false;
   }
   if (buf->b_start_ffc != *buf->b_p_ff) {