add tests to demonstrate date ordering issue with topo sort

When the timestamp for a commit on a branch is older than the point at
which it is being merged back into master, the topo sort does not
produce the values in the same order as the git command line tool. So
although the results are technically sorted in a correct topologicial
form, they produce incorrect information when checking for the most
recent tag or other information when traversing the results.

This patch adds 2 tests to demonstrate the problem.

Signed-off-by: Doug Hellmann <doug@doughellmann.com>