Fix error display when merging PRs (#29288)

Partially fix #29071, regression of Modernize merge button #28140 

Fix some missing `Redirect` -> `JSONRedirect`.

Thanks @yp05327 for the help in
#29071 (comment)

routers/web/repo/pull.go

@@ -1283,19 +1283,19 @@ func MergePullRequest(ctx *context.Context) {
  return
  }
  ctx.Flash.Error(flashError)
- ctx.Redirect(issue.Link())
+ ctx.JSONRedirect(issue.Link())
  } else if models.IsErrMergeUnrelatedHistories(err) {
  log.Debug("MergeUnrelatedHistories error: %v", err)
  ctx.Flash.Error(ctx.Tr("repo.pulls.unrelated_histories"))
- ctx.Redirect(issue.Link())
+ ctx.JSONRedirect(issue.Link())
  } else if git.IsErrPushOutOfDate(err) {
  log.Debug("MergePushOutOfDate error: %v", err)
  ctx.Flash.Error(ctx.Tr("repo.pulls.merge_out_of_date"))
- ctx.Redirect(issue.Link())
+ ctx.JSONRedirect(issue.Link())
  } else if models.IsErrSHADoesNotMatch(err) {
  log.Debug("MergeHeadOutOfDate error: %v", err)
  ctx.Flash.Error(ctx.Tr("repo.pulls.head_out_of_date"))
- ctx.Redirect(issue.Link())
+ ctx.JSONRedirect(issue.Link())
  } else if git.IsErrPushRejected(err) {
  log.Debug("MergePushRejected error: %v", err)
  pushrejErr := err.(*git.ErrPushRejected)