tests/test-convert-bzr-treeroot.t

git-hgext: adjust to the lack of `changelog.heads` method

We don't have a `heads` method returning nodeid, but this is very easy to get
the same result.

This was flagged by pytype.

We can note that the fact this code did not break is probably a good sign that
it is dead code.

However this is a question outside of the scop of this series.

#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 ..