git-diff-tree --stdin: show all parents.

git-diff-tree --stdin: show all parents.

git-diff-tree --stdin ignored second and subsequent parents when
fed git-rev-list --parents output.  Update diff_tree_commit()
function to take a commit object, and pass a fabricated commit
object after grafting the fake parents from diff_tree_stdin().

Signed-off-by: Junio C Hamano <junkio@cox.net>