Fixed missing parameters to gitlab PR create.

Author: chhsia0

scm/driver/gitlab/pr.go

@@ -56,6 +56,8 @@ func (s *pullService) Create(ctx context.Context, repo string, input *scm.PullRe
 	in := url.Values{}
 	in.Set("title", input.Title)
 	in.Set("description", input.Body)
+	in.Set("source_branch", input.Source)
+	in.Set("target_branch", input.Target)
 	path := fmt.Sprintf("api/v4/projects/%s/merge_requests?%s", encode(repo), in.Encode())
 	out := new(pr)
 	res, err := s.client.do(ctx, "POST", path, nil, out)

scm/driver/gitlab/pr_test.go

@@ -155,7 +155,9 @@ func TestPullCreate(t *testing.T) {
 
 	gock.New("https://gitlab.com").
 		Post("/api/v4/projects/diaspora/diaspora/merge_requests").
-		Reply(200).
+		MatchParam("source_branch", "fix").
+		MatchParam("target_branch", "master").
+		Reply(201).
 		Type("application/json").
 		SetHeaders(mockHeaders).
 		File("testdata/merge.json")