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.

hgext/rebase.py

@@ -516,6 +516,12 b' def rebasenode(repo, rev, p1, state, col'
             if state.get(p.rev()) == repo[p1].rev():
                 base = p.node()
                 break
+        else: # fallback when base not found
+            base = None
+
+            # Raise because this function is called wrong (see issue 4106)
+            raise AssertionError('no base found to rebase on '
+                                 '(rebasenode called wrong)')
     if base is not None:
         repo.ui.debug("   detach base %d:%s\n" % (repo[base].rev(), repo[base]))
     # When collapsing in-place, the parent is the common ancestor, we