git-svn: properly set path for "info" command

git-svn: properly set path for "info" command

canonicalize_path() was previously changed to better
fit SVN 1.5, but it makes the "info" command not match
svn(1) in two places:

  1) URL ended up with a trailing slash when run without an
     argument.

  2) "Path: " was displayed instead of "Path: ." when run
     without an argument.

We will also handle odd cases where a user wants to
get information on a file or directory named "0", too.

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