Fix rename branch 500 when the target branch is deleted but exist in database (#30430) (#30437)

Backport #30430 by @lunny

Fix #30428

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

routers/web/repo/setting/protected_branch.go

@@ -312,7 +312,13 @@ func RenameBranchPost(ctx *context.Context) {
 
 	msg, err := repository.RenameBranch(ctx, ctx.Repo.Repository, ctx.Doer, ctx.Repo.GitRepo, form.From, form.To)
 	if err != nil {
-		ctx.ServerError("RenameBranch", err)
+		switch {
+		case git_model.IsErrBranchAlreadyExists(err):
+			ctx.Flash.Error(ctx.Tr("repo.branch.branch_already_exists", form.To))
+			ctx.Redirect(fmt.Sprintf("%s/branches", ctx.Repo.RepoLink))
+		default:
+			ctx.ServerError("RenameBranch", err)
+		}
 		return
 	}