Generate r0 in FSFS to the degree possible without having a proper txn.

Add a regression test that verifies we still end up with the same
quirky content as before.

* subversion/libsvn_fs_fs/fs_fs.c
  (construct_r0_noderev,
   construct_r0_changes): New utilities.
  (write_revision_zero): Use the new utilities to piece r0 together
                         from its parts.  Avoid literals if feasible.

* subversion/tests/libsvn_fs_fs/fs-fs-pack-test.c
  (compare_r0_template): New test.
  (test_funcs): Register new test.