upgrade: more standard creation of the temporary repository
By using the standard path to create a repository we fill some hole in the
current initialization process. The one who triggered this changeset was the
lack of extensions initialization.
$ 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