tests/test-convert-bzr-treeroot.t

histedit: remove an unnecessary default value of `b''` for commit message

I don't think `ctx.description()` is ever anything falsy other than
`b''`. I think the comment added in 4c4232e51167 (histedit: extract
common summary code into method, 2016-05-27) is mistaken (I don't see
any code following that pattern before the function was added).

Differential Revision: https://phab.mercurial-scm.org/D12406

#require bzr

  $ . "$TESTDIR/bzr-definitions"
  $ cat > treeset.py <<EOF
  > import sys
  > from breezy import workingtree
  > import breezy.bzr.bzrdir
  > wt = workingtree.WorkingTree.open('.')
  > 
  > message, rootid = sys.argv[1:]
  > wt.set_root_id(b'tree_root-%s' % rootid.encode())
  > wt.commit(message)
  > EOF

change the id of the tree root

  $ mkdir test-change-treeroot-id
  $ cd test-change-treeroot-id
  $ brz init -q source
  $ cd source
  $ echo content > file
  $ brz add -q file
  $ brz commit -q -m 'Initial add'
  $ "$PYTHON" ../../treeset.py 'Changed root' new
  $ cd ..
  $ hg convert source source-hg
  initializing destination source-hg repository
  scanning source...
  sorting...
  converting...
  1 Initial add
  0 Changed root
  $ manifest source-hg tip
  % manifest of tip
  644   file

  $ cd ..