Traditionally, Gitaly used to return successfully in case access checks
failed for either UserMergeBranch or UserMergeToRef, but it notified the
client that an error has occurred by setting the `pre_receive_error`
field. This has been changed upstream in Gitaly:

    - UserMergeBranch has been converted to make use of the rich gRPC
      error model and thus returns a proper error in case access checks
      fail.

    - UserMergeToRef has been converted to not do access checks at all
      anymore.

As a result, neither of those RPCs will ever return successfully with
the `pre_receive_error` field being set anymore. Gitaly has thus
deprecated those fieilds, which will soon go away completely.

Adapt the Gitaly client to not check for these fields anymore to be
prepared for the removal.