Do not support "partial URL shorthand" anymore.

Do not support "partial URL shorthand" anymore.

We used to support specifying the top part of remote URL in
remotes and use that as a short-hand for the URL.

	$ cat .git/remotes/jgarzik
	URL: git://git.kernel.org/pub/scm/linux/kernel/git/jgarzik/
	$ git pull jgarzik/misc-2.6

This is confusing when somebody attempts to do this:

	$ git pull origin/foo

which is not syntactically correct (unless you have origin/foo.git
repository) and should fail, but it resulted in a mysterious
access to the 'foo' subdirectory of the origin repository.

Which was what it was designed to do, but because this is an
oddball "feature" I suspect nobody uses, let's remove it.

Signed-off-by: Junio C Hamano <junkio@cox.net>