[PATCH] three --merge-order bug fixes

[PATCH] three --merge-order bug fixes

This patch fixes three bugs in --merge-order support
    * mark_ancestors_uninteresting was unnecessarily exponential which
      caused a problem when a commit with no parents was merged near the
      head of something like the linux kernel
    * removed a spurious statement from find_base which wasn't
      apparently causing problems now, but wasn't correct either.
    * removed an unnecessarily strict check from find_base_for_list
      that causes a problem if git-rev-list commit ^parent-of-commit
      is specified.
    * added some unit tests which were accidentally omitted from
      original merge-order patch

The fix to mark_ancestors_uninteresting isn't an optimal fix - a full
graph scan will still be performed in this case even though it is
not strictly required. However, a full graph scan is linear
and still no worse than git-rev-list HEAD which runs in less than 2
seconds on a warm cache.

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>