git-svn: Abort with an error if 'fetch' parameter is invalid.

git-svn: Abort with an error if 'fetch' parameter is invalid.

Previously, if a config entry looked like this:

         svn-remote.svn.fetch=:refs/heads/whatever

git-svn would silently do nothing if you asked it to "git svn fetch", and
give a strange error if asked to "git svn dcommit".  What it really wants is
a line that looks like this:

	svn-remote.svn.fetch=:refs/remotes/whatever

So we should simply abort if we get the wrong thing.

On the other hand, there's actually no good reason for git-svn to enforce
using the refs/remotes namespace, but the code seems to have hardcoded this
in several places and I'm not brave enough to try to fix it all right now.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>