git-rev-list: allow arbitrary head selections, use git-rev-tree syntax

git-rev-list: allow arbitrary head selections, use git-rev-tree syntax

This makes git-rev-list use the same command line syntax to mark the
commits as git-rev-tree does, and instead of just allowing a start and
end commit, it allows an arbitrary list of "interesting" and "uninteresting"
commits.

For example, imagine that you had three branches (a, b and c) that you
are interested in, but you don't want to see stuff that already exists
in another persons three releases (x, y and z). You can do

	git-rev-list a b c ^x ^y ^z

(order doesn't matter, btw - feel free to put the uninteresting ones
first or otherwise swithc them around), and it will show all the
commits that are reachable from a/b/c but not reachable from x/y/z.

The old syntax "git-rev-list start end" would not be written as
"git-rev-list start ^end", or "git-rev-list ^end start".

There's no limit to the number of heads you can specify (unlike
git-rev-tree, which can handle a maximum of 16 heads).