t6012: make rev-list tests more interesting

t6012: make rev-list tests more interesting

As we are working to rewrite some of the revision-walk machinery,
there could easily be some interesting interactions between the
options that force topological constraints (--topo-order,
--date-order, and --author-date-order) along with specifying a
path.

Add extra tests to t6012-rev-list-simplify.sh to add coverage of
these interactions. To ensure interesting things occur, alter the
repo data shape to have different orders depending on topo-, date-,
or author-date-order.

When testing using GIT_TEST_COMMIT_GRAPH, this assists in covering
the new logic for topo-order walks using generation numbers. The
extra tests can be added indepently.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>