Prevent the serialization buffer for changed paths lists from growing
by default.  Pre-allocate it properly with enough space for our change_t
instances which became quite obese since the last release or so.

If necessary, the buffer will still auto-grow.

* subversion/libsvn_fs_fs/temp_serializer.c
  (svn_fs_fs__serialize_changes): 144 bytes for the struct, 106 for the
                                  path(s) - on average.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1604947 13f79535-47bb-0310-9956-ffa450edef68