git-rev-list: use proper lazy reachability analysis

git-rev-list: use proper lazy reachability analysis

This mean sthat you can give a beginning/end pair to git-rev-list,
and it will show all entries that are reachable from the beginning
but not the end.

For example

	git-rev-list v2.6.12-rc5 v2.6.12-rc4

shows all commits that are in -rc5 but are not in -rc4.