Add DefaultMergeStyle option to repository (#14789)

Fixes #12293

routers/api/v1/repo/repo.go

@@ -731,6 +731,7 @@ func updateRepoUnits(ctx *context.APIContext, opts api.EditRepoOption) error {
 					AllowSquash:               true,
 					AllowManualMerge:          true,
 					AutodetectManualMerge:     false,
+					DefaultMergeStyle:         models.MergeStyleMerge,
 				}
 			} else {
 				config = unit.PullRequestsConfig()
@@ -757,6 +758,9 @@ func updateRepoUnits(ctx *context.APIContext, opts api.EditRepoOption) error {
 			if opts.AutodetectManualMerge != nil {
 				config.AutodetectManualMerge = *opts.AutodetectManualMerge
 			}
+			if opts.DefaultMergeStyle != nil {
+				config.DefaultMergeStyle = models.MergeStyle(*opts.DefaultMergeStyle)
+			}
 
 			units = append(units, models.RepoUnit{
 				RepoID: repo.ID,