tests/repo_builder: needs the ability to create mulltiple commits with graph information

I'd like something like this to work:

# A -> B
# A -> C - > D
# A =- merge-base; B = new/debian; C = old/debian; D = old/ubuntu
broken_graph = [
    Commit(
 SourceTree(Source(SourceSpec(version='1'))),
 name='A',
    ),
    Commit(
 SourceTree(Source(SourceSpec(changelog_versions=['1', '2']))),
 name='B',
 parents=['A'],
    ),
    Commit(
 SourceTree(Source(SourceSpec(changelog_versions=['1', '1.1']))),
 name='C',
 parents=['A'],
    ),
]

This needs me to write three features:

SourceSpec/SourceFiles changelog_versions feature.
The SourceTree class feature.
Commit parenting via Placeholder feature.