fix: add extra field to fixed revisions to avoid creating obsolescence cycles The extra field prevents sequential invocations of fix from producing the same hash twice. Previously, this could cause problems because it would create an obsolescence cycle instead of the expected new successor. This change also adds an explicit check for whether a new revision should be committed. Until now, the code relied on memctx.commit() to quietly do nothing if the node already exists. Because of the new extra field, this no longer covers the case where we don't want to replace an unchanged node. Differential Revision: https://phab.mercurial-scm.org/D5245