With the instance ID mechanism in place, we can now take out locks on
source *and* target repo.  Hence, we should block the target repo for
any kinds of modification during the hotcopy process.

* subversion/libsvn_fs_fs/hotcopy.c
  (svn_fs_fs__hotcopy): Grab all target repo locks.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1623379 13f79535-47bb-0310-9956-ffa450edef68