deltas: add a `debug.revlog.debug-delta` config option enable output
This provide a way to enable the code introduced in the previous changeset. This
will provide a large amount of output when applying a bundle with details about
each delta "computation".
$ LANG=nonexistent LC_ALL=nonexistent LANGUAGE=nonexistent hg version -q
Mercurial Distributed SCM (version *) (glob)