git-svn: remove leading slashes from fetch lines in the generate config

git-svn: remove leading slashes from fetch lines in the generate config

We were previously sensitive to leading slashes in the fetch
lines and incorrectly writing them to the config if the user
used them (needlessly) in the command-line.

This fixes the issue and allows us to play nicely with legacy
configs that have leading slashes in fetch lines.

Thanks to Bradford Smith for figuring this out for me:
>
> This works:
>
> git-svn clone https://my.server.net/repos/path/ -Ttrunk/testing
>   -ttags/testing -bbranches/testing testing
>
> This doesn't:
>
> git-svn clone https://my.server.net/repos/path -T/trunk/testing
>   -t/tags/testing -b/branches/testing testing

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