Show difference in commit messages in two different git branches using Bitbucket REST API

bitbucket-git-branch-diff-msg.cs

using Newtonsoft.Json;
using RestSharp;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Foo
{
    public interface IBitbucketService
    {
        string GetDiffMessages(string sourceBranch, string destinationBranch, string fields, string pagelen);
    }

    public class BitbucketService : IBitbucketService
    {
        public string GetDiffMessages(string sourceBranch, string destinationBranch, string fields, string pagelen)
        {
            // reference - https://developer.atlassian.com/bitbucket/api/2/reference/resource/repositories/%7Busername%7D/%7Brepo_slug%7D/commits

            var url = "https://api.bitbucket.org/2.0/repositories/mycompany/myproject/commits/";
            var client = new RestClient(url);
            var req = new RestRequest(Method.GET);

            if(fields == null || fields.Length == 0)
                fields = "values.message,next";

            if (string.IsNullOrEmpty(pagelen))
                pagelen = "100";

            req.AddQueryParameter("access_token", "<my_access_token>");
            req.AddQueryParameter("fields", fields);
            req.AddQueryParameter("include", sourceBranch);
            req.AddQueryParameter("exclude", destinationBranch);
            req.AddQueryParameter("pagelen", pagelen);

            var access_token = GetAccessToken();
            req.AddHeader("Authorization", $"Bearer {access_token}");

            return client.Execute(req).Content;
        }

        private string GetAccessToken()
        {
            var url = "https://bitbucket.org/site/oauth2/access_token";
            var client = new RestClient(url);
            var req = new RestRequest(Method.POST);

            req.AddHeader("Content-Type", "application/x-www-form-urlencoded");
            req.AddHeader("Authorization", "Basic <my_token>");

            req.AddParameter("application/x-www-form-urlencoded", "grant_type=client_credentials", ParameterType.RequestBody);

            dynamic res = JsonConvert.DeserializeObject(client.Execute(req).Content);

            return res.access_token;
        }
    }
}