mercurial/templates/rss/changelog.tmpl

merge: check argument value with if/raise instead of an assert

This shouldn't make any difference for legal code, but it'll prevent
the assertion from being optimized out if someone decides to do -O on
our code.

Differential Revision: https://phab.mercurial-scm.org/D6879

{header}
    <title>{repo|escape} Changelog</title>
    <description>{repo|escape} Changelog</description>
    {entries%changelogentry}
  </channel>
</rss>