convert: add a config option to help doing identity hg->hg conversion I want to change the computation of the list of files modified by a commit. In principle, this would simply change a cache. But since this information is stored in commits rather than a cache, changing it means changing commit hashes (going forward). Some users rely on the convert extension from hg to hg not changing hashes when nothing changes (usually). Allow these users to preserve hashes despite changes to the changelog files computation by reusing these files lists when the manifest is unchanged (since these files list are derived from the manifest). Differential Revision: https://phab.mercurial-scm.org/D6643