discovery: prevent crash caused by prune marker having no parent data
If a marker has no parent information, parents field is set to None, which
caused TypeError. I think this shouldn't normally happen, but somehow
I have such marker in my repo.
changeset: {node}
user: {user}
date: {age}
summary: {desc}
{branches%branchname}{tags%tagname}{bookmarks%bookmarkname}
node: ({col}, {row}) (color {color})
{edges%graphedge}