git-svn: Fix branch detection when repository root is inaccessible

git-svn: Fix branch detection when repository root is inaccessible

For the case of multiple projects sharing a single SVN repository, it is
common practice to create the standard SVN directory layout within a
subdirectory for each project. In such setups, access control is often
used to limit what projects a given user may access. git-svn failed to
detect branches (e.g. when passing --stdlayout to clone) because it
relied on having access to the root directory in the repository. This
patch solves this problem by making git-svn use paths relative to the
given repository URL instead of the repository root.

Signed-off-by: Mattias Nissler <mattias.nissler@gmx.de>
Acked-by: Eric Wong <normalperson@yhbt.net>