ancestor: add a way to remove ancestors of bases from a given set
This and missingancestors can share state, which will turn out to be perfect
for set discovery.
{header}
<title>Mercurial repository not found</title>
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<body>
<h2>Mercurial repository not found</h2>
The specified repository "{repo|escape}" is unknown, sorry.
Please go back to the <a href="{url|urlescape}">main repository list page</a>.
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