prepush: fix bug in warning message selection
In certain situations you would be told "you should pull and merge" even when
there is nothing to pull.
#!/bin/shhg initecho a > ahg ci -Am thg mv a bhg ci -Am t1hg debugrename bhg mv b ahg ci -Am t2hg debugrename aecho % test with --revhg debugrename --rev 1 b