combine-diff: fix appending at the tail of a list.
combine-diff: fix appending at the tail of a list.

... and use the established pattern of tail initialized to point
at the head pointer for an empty list, and updated to point at
the next pointer field of the item at the tail when appending.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git