rollback: detect "parentgone" case earlier
Detecting this earlier will help us to affect the rollback process sooner, which
will help the next changesets.
To keep things simple, we degrade the behavior a bit when the `undo.desc` is
missing. However since `hg rollback` is not supposed to be used and the
`undo.desc` file have been introduced in mercurial 1.6 (2010). I think this is
an acceptable evil.
{
"version": 1,
"project": "mercurial",
"project_url": "https://mercurial-scm.org/",
"repo": "..",
"branches": ["default", "stable"],
"environment_type": "virtualenv",
"show_commit_url": "https://www.mercurial-scm.org/repo/hg/rev/",
"benchmark_dir": "benchmarks",
"env_dir": "../.asv/env",
"results_dir": "../.asv/results",
"html_dir": "../.asv/html"
}