revlogv2: delay the update of the changelog docket to transaction end
This prevent external reader to see the transaction content before it is
commited. However this also prevent the hooks to see the transaction content.
We will fix this in later changesets.
We have to temporarily suppress the error output of the command ran during the
transaction as they sometimes get confused about unknown working directory and
sometimes issue message on std-err in unspecified order.
Differential Revision: https://phab.mercurial-scm.org/D10629