debugformat: update label depending on value difference The new label highlight areas where the repo format differs from current config or default. This should help people spot area where a repository mismatch with the expected state.

debugformat: add data about the config when verbose In verbose mode, the command also displays the current configuration choice for the variant and the global Mercurial default for it.

debugformat: add a 'debugformat' command The command displays basic data about all format variants registered for repo upgrades. This gives a quick way to peek into a repository format. The 'fm.write()' calls are very independent because more data will be added in later changeset. Having more separate call make the later patch clearer.