upgrade: speed up when we have only nodemap to downgrade
Similar to what we do on upgrade, if we have only persistent-nodemap to
downgrade we will just delete the nodemap files and update repository
requirements instead of processing all the revlogs.
After downgrade, we are left with unrequired docket and transaction files which
seems fine but can work on deleting them if someone feels we should.
Differential Revision: https://phab.mercurial-scm.org/D9992
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