In FSFS, use a sub-pool in the functions that call get_dag and need to
lock the cache until those functions complete.  The sub-pool's destruction
will release the cache locks implicitly.

As a side-benefit, we get a nicer memory usage profile because all these
functions lack a separate scratch pool parameter.

* subversion/libsvn_fs_fs/tree.c
  (fs_props_changed,
   fs_copy,
   fs_revision_link,
   fs_contents_changed,
   fs_get_file_delta_stream): Create a sub-pool and use that as a scratch pool
                              for all non-result data.  Destroy that pool upon
                              successful completion of the function.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1648243 13f79535-47bb-0310-9956-ffa450edef68