graft: use missing ancestors algorithm to find earlier grafts
When the revisions to graft are numerically close to the destination, this
avoids two walks up the DAG, which for a repository with over 470,000
changesets translates to around 2.2 seconds.
{header}
<title>{repo|escape}: {file|escape} history</title>
<description>{file|escape} revision history</description>
{entries%filelogentry}
</channel>
</rss>