revision.c: fix "dense" under --remove-empty

It had the wrong test for whether a commit was a merge. What it did was to
say that a non-merge has exactly one parent (which sounds almost right),
but the fact is, initial trees have no parent at all, but they're
obviously not merges.
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

revision.c

@@ -997,7 +997,7 @@ struct commit *get_revision(struct rev_info *revs)
 				if (!revs->parents)
 					continue;
 				/* non-merge - always ignore it */
-				if (commit->parents && !commit->parents->next)
+				if (!commit->parents || !commit->parents->next)
 					continue;
 			}
 			if (revs->parents)