upgrade: take lock only for part where it's required
The final config calculation code does not require a lock, only writing it back
does require one.
Differential Revision: https://phab.mercurial-scm.org/D9783
$ 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