Explicitly define author in merge test commit and introduced merge method to IRepository.

Author: angusbjones

LibGit2Sharp.Tests/MergeFixture.cs

@@ -104,7 +104,7 @@ public void CanMergeRepos()
 
                 MergeResult mergeResult = repo.Merge(repo.Branches["FirstBranch"].Tip);
 
-                var mergeCommit = repo.Commit("Merge First+Second");
+                var mergeCommit = repo.Commit("Merge First+Second", Constants.Signature, Constants.Signature);
 
                 Assert.Equal(mergeCommit.Tree.Count, originalTreeCount + 3);    //Expecting original tree count plussed by the 3 added files.
                 Assert.Equal(mergeCommit.Parents.Count(), 2);   //Merge commit should have 2 parents

LibGit2Sharp/IRepository.cs

@@ -193,6 +193,11 @@ public interface IRepository : IDisposable
         /// </summary>
         IEnumerable<MergeHead> MergeHeads { get; }
 
+        /// <summary>
+        /// Merges the given commit into HEAD.
+        /// </summary>
+        MergeResult Merge(Commit commit);
+
         /// <summary>
         /// Manipulate the currently ignored files.
         /// </summary>