git-svn: fix rel_path() when not connected to the repository root

git-svn: fix rel_path() when not connected to the repository root

This should fix fetching for people who did not use
"git svn --minimize" or cannot connect to the repository root
due to the lack of permissions.

I'm not sure what I was on when I made the change to the
rel_path() function in 4e9f6cc78e5d955bd0faffe76ae9aea6590189f1
that made it die() when we weren't connected to the repository
root :x

Thanks to Sven Verdoolaege for reporting this bug.

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