convert: use repo.lookup() for converting to nodeid

This is a list of revs that come from the CLI (opts['rev']). Perhaps
we should allow any revset, but I'll leave that for someone else to
improve if they care.

Differential Revision: https://phab.mercurial-scm.org/D3089

hgext/convert/hg.py

@@ -481,7 +481,7 b' class mercurial_source(common.converter_'
             else:
                 self.keep = util.always
             if revs:
-                self._heads = [self.repo[r].node() for r in revs]
+                self._heads = [self.repo.lookup(r) for r in revs]
             else:
                 self._heads = self.repo.heads()
         else: