D857: py3: add a test showing a normal `hg merge` works on Python 3

[durin42 (Augie Fackler)]

durin42 added a comment.


  Do we need this with https://phab.mercurial-scm.org/D858? It looks like we get enough extra coverage from that we don't need this extra check here.

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D857

To: pulkit, #hg-reviewers
Cc: durin42, mercurial-devel