localrepo: validate directories before creating any
There is no meaningful change in behavior because wdir would
already exist in the case where we raised RepoError. But I think
the code is easier to read if we do all validation first then
take actions with side-effects.
Differential Revision: https://phab.mercurial-scm.org/D4706
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}