This corner-case was triggered by a kernel commit that was not in date
order, due to a misconfigured time zone that made the commit appear three
hours older than it was.

That caused git-rev-list to traverse the commit tree in a non-obvious
order, and made it parse several of the _parents_ of the misplaced commit
before it actually parsed the commit itself. That's fine, but it meant
that the grandparents of the commit didn't get marked uninteresting,
because they had been reached through an "interesting" branch.

The reason was that "mark_parents_uninteresting()" (which is supposed to
mark all existing parents as being uninteresting - duh) didn't actually
traverse more than one level down the parent chain.

NORMALLY this is fine, since with the date-based traversal order,
grandparents won't ever even have been looked at before their parents (so
traversing the chain down isn't needed, because the next time around when
we pick out the parent we'll mark _its_ parents uninteresting), but since
we'd gotten out of order, we'd already seen the parent and thus never got
around to mark the grandparents.

Anyway, the fix is simple. Just traverse parent chains recursively.
Normally the chain won't even exist (since the parent hasn't been parsed
yet), so this is not actually going to trigger except in this strange
corner-case.

Add a comment to the simple one-liner, since this was a bit subtle, and I
had to really think things through to understand how it could happen.
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>