nodemap: deal with the "debugupdatecache" case using a "fake" transaction
We are going to need more and more methods of the transaction. So lets change
approach. The hg debugupdatecache case do not need a transaction, because has
the repositories locked, but is not adding any "store" data to it.
Differential Revision: https://phab.mercurial-scm.org/D8186