GetReflog uses Repository#getReflogReader, but this is not implemented
by DfsRepository which is used in the test framework. As a result, calls
to this API method from tests results in UnsupportedOperationException.
Modify GetReflog to catch the UOE and rethrow as MethodNotAllowed, so
we get a graceful failure rather than "internal server error".
In the tests, expect MethodNotAllowed, with a TODO to rework when/if
the implementation of getReflogReader is done in DfsRepository.
Change-Id: I0d0d718ca4e4ecf2c544ea6e912397c20e2fd7e3
419ed573b9