rust: add comment about lack of wdirrev handling
If hg is compiled with rust support, 'only(wdir())' crashed as
"rustext.GraphError: ('ParentOutOfRange',
2147483647)", which should instead
say "abort: working directory revision cannot be specified."
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state