git-svn.perl: workaround assertions in svn library 1.5.0

git-svn.perl: workaround assertions in svn library 1.5.0

With subversion 1.5.0 (C and perl libraries) the git-svn selftest
t9101-git-svn-props.sh fails at test 25 and 26.  The following commands
cause assertions in the svn library

 $ cd deeply
 $ git-svn propget svn:ignore .
 perl: /build/buildd/subversion-1.5.0dfsg1/subversion/libsvn_ra/ra_loader.c:674: svn_ra_get_dir: Assertion `*path != '/'' failed.
 Aborted

 $ git-svn propget svn:ignore ..
 perl: /build/buildd/subversion-1.5.0dfsg1/subversion/libsvn_subr/path.c:120: svn_path_join: Assertion `is_canonical(component, clen)' failed.

With this commit, git-svn makes sure the path doesn't start with a
slash, and is not a dot, working around these assertions.

The breakage was reported by Lucas Nussbaum through
 http://bugs.debian.org/489108

Signed-off-by: Gerrit Pape <pape@smarden.org>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>