avoid git syncing with instantiate proxied remotes

avoid git syncing with instantiate proxied remotes

These remotes have no url configured, so git pull and push will fail.
git-annex sync --content etc can still sync with them otherwise.

Also, avoid git syncing twice with the same url. This is for cases where
a proxied remote has been manually configured and so does have a url.
Or perhaps proxied remotes will get configured like that automatically
later.