update documentation and handle when repo is null

Author: paladique

src/GitHub.Exports/Services/GitService.cs

@@ -77,13 +77,16 @@ public UriString GetRemoteUri(IRepository repo, string remote = "origin")
         /// Finds the latest pushed commit of a file and returns the sha of that commit. Returns null when no commits have 
         /// been found in any remote branches or the current local branch. 
         /// </summary>
-        /// <param name="path">The local path of the file. This cannot be null.</param>
+        /// <param name="path">The local path of a repository or a file inside a repository. This cannot be null.</param>
         /// <returns></returns>
         public Task<string> GetLatestPushedSha(string path)
         {
             Guard.ArgumentNotNull(path, nameof(path));
             var repo = GetRepository(path);
 
+            if (repo == null)
+                return null;
+
             if (repo.Head.IsTracking && repo.Head.Tip.Sha == repo.Head.TrackedBranch.Tip.Sha)
             {
                 return Task.FromResult(repo.Head.Tip.Sha);

src/GitHub.Exports/Services/IGitService.cs

@@ -46,6 +46,12 @@ public interface IGitService
         /// <returns></returns>
         UriString GetRemoteUri(IRepository repo, string remote = "origin");
 
+        /// <summary>
+        /// Finds the latest pushed commit of a file and returns the sha of that commit. Returns null when no commits have 
+        /// been found in any remote branches or the current local branch. 
+        /// </summary>
+        /// <param name="path">The local path of a repository or a file inside a repository. This cannot be null.</param>
+        /// <returns></returns>
         Task<string> GetLatestPushedSha(string path);
     }
 }