Add revlog.reachable to find a graph of ancestors for a given rev

[PATCH] raise exceptions with Exception subclasses Fixed the patch. Using Exception subclasses. (tweaked by mpm)

Fixes for push corner case TAH found a bug where push would push things the remote already had, raising an assertion. This turned out to be because the changeset protocol was not recording a common subset node in one case. Also discovered was that the protocol was queueing multiple copies of a node for pull. Fixed by changing fetch to a hash. Add some more debugging output.