merge: make test for fast-forward merge stricter (issue619)
don't allow merging with an ancestor
fix != on contexts
add a test
#!/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