[PATCH] rev-tree support for "in X but not in Y".

[PATCH] rev-tree support for "in X but not in Y".

To do the automated commit-mailing I need to be able to answer the
question "which commits are here today but weren't yesterday"...  i.e.
given two commit-ids $HEAD and $YESTERDAY I want to be able to do:

	rev-tree $HEAD ^$YESTERDAY

to list those commits which are in the tree now but weren't
ancestors of yesterday's head.

Yes, I could probably do this with
	rev-tree $HEAD $YESTERDAY | egrep -v ^[a-z0-9]*:3
but I prefer not to.