debugcommands: introduce debugrevlogindex (BC) `hg debugindex` was originally invented for displaying revlog index data and is highly tailored towards that. e.g. it accepts a --format option to display index data for a particular revlog version and displays things like offset and length. As we support non-revlog storage, it makes sense for `hg debugindex` to display generic index data and for there to exist storage-specific or storage-aware debug* commands for dumping non-generic index data. This commit effectively renames `hg debugindex` to `hg debugrevlogindex` and replaces `hg debugindex` with a version that is storage agnostic. Tests using --format have been updated to use `hg debugrevlogindex`. Output is backwards compatible. The replacement command uses the formatter, which means output can be templatized. At some point, we may want to tweak output (e.g. to add the revision size). But I don't feel like taking a bigger BC break at the moment. The renamed command/function had to be moved because check-code enforces alphabetical ordering of commands in this file. Differential Revision: https://phab.mercurial-scm.org/D4358