If there's a git remote named "foo", and a special remote with the same
name, `git annex enableremote foo` just prints "ok" and silently does
nothing.

Perhaps worse, passing enableremote the uuid of the special remote causes
it to *modify* the same named git remote's config, adding/replacing
annex-uuid and adding whatever other git configs the special remote uses.
--[[Joey]]

(initremote does check if there's already a remote with the same name, and
refuses to overwrite it).
