When we create a FSFS transaction, we are the first ones to set revprops
on it.  So, let's write them directly to disk instead of trying to read
previous file contents.

The new code does not only avoid the creation of expensive error objects
but also introduces setter for revprops as a symmetric counterpart to
get_txn_proplist.

* subversion/libsvn_fs_fs/transaction.c
  (set_txn_proplist): new function with code taken from ...
  (svn_fs_fs__change_txn_props): ... this one; call new function
  (svn_fs_fs__begin_txn): set initial revprops instead of updating them

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1544719 13f79535-47bb-0310-9956-ffa450edef68