treemanifest: rework lazy-copying code (issue4840) The old lazy-copy code formed a chain of copied manifests with each copy. Under typical operation, the stack never got more than a couple of manifests deep and was fine. Under conditions like hgsubversion or convert, the stack could get hundreds of manifests deep, and eventually overflow the recursion limit for Python. I was able to consistently reproduce this by converting an hgsubversion clone of svn's history to treemanifests. This may result in fewer manifests staying in memory during operations like convert when treemanifests are in use, and should make those operations faster since there will be significantly fewer noop function calls going on. A previous attempt (never mailed) of mine to fix this problem tried to simply have all treemanifests only have a loadfunc - that caused somewhat weird problems because the gettext() callable passed into read() wasn't idempotent, so the easy solution is to have a loadfunc and a copyfunc.