
* Removed `SvnAccess.update_after_write` knob and instead set default value of True
  to the `update_after_write` argument to `SvnAccess.write`; callers can implement
  alternatives trivially by subclassing.

* Added `update_before_write` argument to .write with default value of True. Turn
  this off as well as `update_after_write` if you want to manage synchronicity in
  exchange for potentially significant performance improvements.

* Added `default_commit_message` argument to `SvnAccess` constructor. The default
  is "foom".

* Throw `exc.ResourceChanged` exceptions when a file is found to be out of date
  during a write operation (which will only happen if `update_before_write` is
  unset by the caller) and revert local changes if this happens, to restore the
  checkout to an unconflicting state.
