Currently test_merge() only allows developers to merge in one branch.
However, this restriction is artificial and there is no reason why it
needs to be this way.

Extend test_merge() to allow the specification of multiple branches so
that octopus merges can be performed.
Signed-off-by: NDenton Liu <liu.denton@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>