children extension: Don't abort when looking at the null revision.
Since 62c56d8f368b revlog.children only returns real children,
so this is no longer a problem.
#!/bin/sh
hg init
echo a > a
hg ci -Am t
hg mv a b
hg ci -Am t1
hg debugrename b
hg mv b a
hg ci -Am t2
hg debugrename a
echo % test with --rev
hg debugrename --rev 1 b