hg-ssh: reject push earlier (on pretxnopen) We now have a lock triggered for any transaction. We use it to ensure no-read are made in read-only mode. We need more that just "no changegroup is added", since bundle2 allows for more than just changegroup to be exchanged. We still protect pushkey as it may write data without opening a transaction.

test: test both bundle1 and bundle2 paths in 'test-push-hook-lock.t' The locking scheme is a bit different in each cases. So we explicitly test each case.