obsutil: move 'allprecursors' to the new modules
We have a new 'obsutil' module now. We move the high level utility there to
bring 'obsolete.py' back to a more reasonable size.
$ 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