Setting a project up on another host (a cloned or imported folder)
only knew how to link the folder through a GitHub identity. A project
keyed by its Git remote (GitLab, Bitbucket, a self-hosted host) was
always refused with "Imported folder does not match the selected
project identity". A fresh clone's remote is not probed when it is
registered, so it never projected into that project by itself.
The link now probes an unprobed folder's remote. If the canonical key
of that remote is the project's, the remote is recorded on the repo, so
the folder projects into the project. SSH and HTTPS forms share a key.
A GitHub project is still linked by stamping upstream, and a remote
that names another repository is still refused.
