Apply sign field when merging

In previous issues(#570022 (closed), #570021 (closed)) a sign field was added to UserMergeRequest and UserMergeToRefRequest. This field was intentionally not being used by gitaly to allow gitlab to start sending it(#578771 (closed), #578772 (closed)).

The UserMerge and UserMergeToRef rpcs needs to be updated to use the sign field to determine whether or not the resulting commit will be signed