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/fileio.c

@@ -2842,7 +2842,7 @@ static int move_lines(buf_T *frombuf, buf_T *tobuf)
   // Copy the lines in "frombuf" to "tobuf".
   curbuf = tobuf;
   for (linenr_T lnum = 1; lnum <= frombuf->b_ml.ml_line_count; lnum++) {
-    char *p = xstrdup(ml_get_buf(frombuf, lnum, false));
+    char *p = xstrdup(ml_get_buf(frombuf, lnum));
     if (ml_append(lnum - 1, p, 0, false) == FAIL) {
       xfree(p);
       retval = FAIL;