[PATCH 1 of 2 V2] debugdeltachain: avoid division by zero when a chain is empty

[Yuya Nishihara]

On Wed, 11 Jul 2018 14:48:34 +0200, Boris Feld wrote:
> # HG changeset patch
> # User Paul Morelle <paul.morelle at octobus.net>
> # Date 1529597997 -7200
> #      Thu Jun 21 18:19:57 2018 +0200
> # Node ID 0be6256636d19c196111d2445b3d6bd8aff5e445
> # Parent  4d5fb4062f0bb159230062701461fa6cab9b539b
> # EXP-Topic debugdeltachain-divbyzero
> # Available At https://bitbucket.org/octobus/mercurial-devel/
> #              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 0be6256636d1
> debugdeltachain: avoid division by zero when a chain is empty

Queued these. Thanks for adding tests for ZeroDivisionError.