diff-highlight: test interleaved parallel lines of history

diff-highlight: test interleaved parallel lines of history

The graph test in t9400 covers the case of two simultaneous
branches, but all of the commits during this time are on the
right-hand branch. So we test a graph structure like:

  | |
  | * commit ...
  | |

but we never see the reverse, a commit on the left-hand
branch:

  | |
  * | commit ...
  | |

Since this is an easy thing to get wrong when touching the
graph-matching code, let's cover it by adding one more
commit with its timestamp interleaved with the other branch.

Note that we need to pass --date-order to convince Git to
show it this way (since --topo-order tries to keep lines of
history separate).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>