rebase: do not raise an UnboundLocalError when called wrong (issue4106) When the base is not found, we should not raise a traceback about a not defined variable. This hides the real problem: the function rebasenode was (probably) called wrong. An AssertionError is raised to highlight that the caller of the function did something wrong. An alternative approach is to only assign None to the variable "base" and let the merge mechanism raise an abort message. This was the behaviour for this case before ad9db007656f. But the only known case for this problem is when an extension calls this function wrong. An AssertionError makes this clearer than an abort message. When a different case is detected, the behaviour can be improved then.