tests: teach verify_parents to check for extra parents

tests: teach verify_parents to check for extra parents

Currently verify_parents only makes sure that the earlier parents of
HEAD match the commits given, and does not care if there are more
parents.  This makes it harder than one would like to check that, for
example, parent reduction works correctly when making an octopus.

Fix it by checking that HEAD^(n+1) is not a valid commit name.
Noticed while working on a new test that was supposed to create a
fast-forward one commit ahead but actually created a merge.

Reported-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>