Fix obsolete comment in set_rel_size().

The cross-reference to set_append_rel_pathlist() was obsoleted by
commit e2fa76d8, which split what
had been set_rel_pathlist() and child routines into two sets of
functions.  But I (tgl) evidently missed updating this comment.

Back-patch to 9.2 to avoid unnecessary divergence among branches.

Amit Langote