When creating L2P and P2L index files allow for a previous failed
commit that created obstructing, read-only, files.

* subversion/libsvn_fs_fs/index.c:
  (index_create): New.
  (svn_fs_fs__l2p_index_create,
   svn_fs_fs__p2l_index_create): Call index_create.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1549924 13f79535-47bb-0310-9956-ffa450edef68