Obtain svn_fs_t via svn_repos_open() -> svn_repos_fs()

Obtain svn_fs_t via svn_repos_open() -> svn_repos_fs()

Without really knowing the svn API, using plain svn_fs_open() complained
about a missing /foo/DB_CONFIG even on fsfs type repos. Use
svn_repos_open() to open the repo and use svn_repos_fs() to get the fs
instead of using svn_fs_open() since even the headers say I wouldn't
want to use it directly... and they're right, obviously. :)

Signed-off-by: Rocco Rutte <pdmef@gmx.net>