sidedata: add a way of replacing an existing sidedata computer
This will be useful in a future patch to replace a sequential computer with
a parallel computer. We only allow for explicit replacement, to force the users
to think about overriding computers.
Differential Revision: https://phab.mercurial-scm.org/D10358
$ LANG=nonexistent LC_ALL=nonexistent LANGUAGE=nonexistent hg version -q
Mercurial Distributed SCM (version *) (glob)