git-rev-parse: add "--not" flag to mark subsequent heads negative

git-rev-parse: add "--not" flag to mark subsequent heads negative

If you have two lists of heads, and you want to see ones reachable from
list $a but not from list $b, just do

	git-rev-list $(git-rev-parse $a --not $b)

which is useful for both bisecting (where "b" would be the list of known
good revisions, and "a" would be the latest found bad head) and for just
seeing what the difference between two sets of heads are if you want to
generate a pack-file for the difference.