Add default merge options when adding new repository (#4369)

models/repo.go

 				Type:   tp,
 				Config: &IssuesConfig{EnableTimetracker: setting.Service.DefaultEnableTimetracking, AllowOnlyContributorsToTrackTime: setting.Service.DefaultAllowOnlyContributorsToTrackTime},
 			})
+		} else if tp == UnitTypePullRequests {
+			units = append(units, RepoUnit{
+				RepoID: repo.ID,
+				Type:   tp,
+				Config: &PullRequestsConfig{AllowMerge: true, AllowRebase: true, AllowSquash: true},
+			})
 		} else {
 			units = append(units, RepoUnit{
 				RepoID: repo.ID,