parsers: fix two leaks in index_ancestors
Both happy paths through this function leaked the returned list:
1) If the list was of size 0 or 1, it was retained an extra time and then
returned.
2) If the list was passed to find_deepest, it was never released before
exiting this function.
Both paths spotted by cpychecker.
$ 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