Make the rev-tree output more regular. This is the last

Make the rev-tree output more regular. This is the last
change. Promise.

It now always outputs all the revisions as <sha1>:<reachability>, where the
reachability is the bitmask of how that revision was reachable from the
commits in the argument list.

Trivially, if there is only one commit, the reachability will always be
(1 << 0) == 1 for all reachable revisions, and there won't be any edges
(so the "--edges" flag only makes sense with multiple commit keys).