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).