ref-log: allow ref@{count} syntax.

ref-log: allow ref@{count} syntax.

Often I find myself wanting to say 'tip of "next" before I
merged the last three topics'.  Now I can say that with:

	git log next@{3}..next

Since small integers alone are invalid input strings to
approxidate, there is no fear of confusion.

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