moved function out to separate client

Octokit/Clients/IRepositoryCommitsClient.cs

@@ -0,0 +1,19 @@
+using System.Diagnostics.CodeAnalysis;
+using System.Threading.Tasks;
+
+namespace Octokit
+{
+    public interface IRepositoryCommitsClient
+    {
+        /// <summary>
+        /// Compare two references in a repository
+        /// </summary>
+        /// <param name="owner">The owner of the repository</param>
+        /// <param name="name">The name of the repository</param>
+        /// <param name="base">The reference to use as the base commit</param>
+        /// <param name="head">The reference to use as the head commit</param>
+        /// <returns></returns>
+        [SuppressMessage("Microsoft.Naming", "CA1716:IdentifiersShouldNotMatchKeywords", MessageId = "base")]
+        Task<CompareResult> Compare(string owner, string name, string @base, string head);
+    }
+}