Possible metadata changes

[glandium]

Filing this to keep around as food for thought:

• group hg2git, git2hg(changeset notes) and files-meta under the same commit, as subtrees, rather than keeping them as separate commits. This would allow easier comparisons between old and new state after an update, where currently one has to compare refs/cinnabar/metadata^6^3 and refs/cinnabar/metadata^3, then refs/cinnabar/metadata^6^4 and refs/cinnabar/metadata^4, then finally refs/cinnabar/metadata^6^5 and refs/cinnabar/metadata^5. An alternative would be git cinnabar subcommands.
• separate hg2git data per hg object type (changeset, manifest, file), which might be worth doing at the same time if and when the above is implemented. That would solve theoretical issues with sha1 conflicts between object types, where the real occurrence of this happens with the empty root manifest/empty new files (and is currently special-cased). Although one thing to consider is how the newly added (and experimental) hg manifest-trees will fit in the picture (it needs consideration anyways).
• reference manifests by their git sha1 in changeset metadata rather than mercurial sha1. The mercurial sha1 is in the git commit with that sha1, and that would avoid a hg2git lookup when looking for the manifest. As a matter of fact, it would probably be possible to not store hg2git lookup info for manifests at all with this change.