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

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: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>