commands: add debugdeltachain command We have debug commands for displaying overall revlog statistics (debugrevlog) and for dumping a revlog index (debugindex). As part of investigating various aspects of revlog behavior and performance, I found it important to have an understanding of how revlog delta chains behave in practice. This patch implements a "debugdeltachain" command. For each revision in a revlog, it dumps information about the delta chain. Which delta chain it is part of, length of the delta chain, distance since base revision, info about base revision, size of the delta chain, etc. The generic formatting facility is used, which means we can templatize output and get machine readable output like JSON. This command has already uncovered some weird history in mozilla-central I didn't know about. So I think it's valuable.

--- a/tests/test-completion.t	Sat Oct 24 19:56:39 2015 +0100
+++ b/tests/test-completion.t	Sat Dec 05 23:37:46 2015 -0800
@@ -80,6 +80,7 @@
   debugdag
   debugdata
   debugdate
+  debugdeltachain
   debugdirstate
   debugdiscovery
   debugextensions
@@ -243,6 +244,7 @@
   debugdag: tags, branches, dots, spaces
   debugdata: changelog, manifest, dir
   debugdate: extended
+  debugdeltachain: changelog, manifest, dir, template
   debugdirstate: nodates, datesort
   debugdiscovery: old, nonheads, ssh, remotecmd, insecure
   debugextensions: template