Fetch via refs/pull/<PR>/head in case branch deleted

Deleted branches can only be fetched by owner of repo.
Fetch via special `refs/pull/<PR>/head` refspec to avoid this issue.

Author: jcansdale

src/GitHub.App/Services/GitClient.cs

@@ -379,6 +379,45 @@ public Task<bool> IsModified(IRepository repository, string path, byte[] content
             });
         }
 
+        public async Task<string> GetPullRequestMergeBase(IRepository repo,
+            UriString targetCloneUrl, string baseSha, string headSha, string baseRef, int pullNumber)
+        {
+            Guard.ArgumentNotNull(repo, nameof(repo));
+            Guard.ArgumentNotNull(targetCloneUrl, nameof(targetCloneUrl));
+            Guard.ArgumentNotEmptyString(baseRef, nameof(baseRef));
+
+            var baseCommit = repo.Lookup<Commit>(baseSha);
+            if (baseCommit == null)
+            {
+                await Fetch(repo, targetCloneUrl, baseRef);
+                baseCommit = repo.Lookup<Commit>(baseSha);
+                if (baseCommit == null)
+                {
+                    throw new NotFoundException($"Couldn't find {baseSha} after fetching from {targetCloneUrl}:{baseRef}.");
+                }
+            }
+
+            var headCommit = repo.Lookup<Commit>(headSha);
+            if (headCommit == null)
+            {
+                var headRef = $"refs/pull/{pullNumber}/head";
+                await Fetch(repo, targetCloneUrl, headRef);
+                headCommit = repo.Lookup<Commit>(headSha);
+                if (headCommit == null)
+                {
+                    throw new NotFoundException($"Couldn't find {headSha} after fetching from {targetCloneUrl}:{headRef}.");
+                }
+            }
+
+            var mergeBaseCommit = repo.ObjectDatabase.FindMergeBase(baseCommit, headCommit);
+            if (mergeBaseCommit == null)
+            {
+                throw new NotFoundException($"Couldn't find merge base between {baseCommit} and {headCommit}.");
+            }
+
+            return mergeBaseCommit.Sha;
+        }
+
         public async Task<string> GetPullRequestMergeBase(IRepository repo,
             UriString baseCloneUrl, UriString headCloneUrl, string baseSha, string headSha, string baseRef, string headRef)
         {

src/GitHub.App/Services/PullRequestService.cs

@@ -313,11 +313,10 @@ public IObservable<string> ExtractFile(
                         sha = await gitClient.GetPullRequestMergeBase(
                             repo,
                             pullRequest.Base.RepositoryCloneUrl,
-                            pullRequest.Head.RepositoryCloneUrl,
                             pullRequest.Base.Sha,
                             pullRequest.Head.Sha,
                             pullRequest.Base.Ref,
-                            pullRequest.Head.Ref);
+                            pullRequest.Number);
                     }
                     catch (NotFoundException ex)
                     {

src/GitHub.Exports.Reactive/Services/IGitClient.cs

@@ -193,13 +193,28 @@ public interface IGitClient
         /// <param name="baseSha">The PR base SHA.</param>
         /// <param name="headSha">The PR head SHA.</param>
         /// <param name="baseRef">The PR base ref (e.g. 'master').</param>
-        /// <param name="pullNumber">The PR number.</param>
+        /// <param name="headRef">The PR head ref.</param>
         /// <returns>
         /// The merge base SHA or null.
         /// </returns>
         /// <exception cref="LibGit2Sharp.NotFoundException">Thrown when the merge base can't be found.</exception>
         Task<string> GetPullRequestMergeBase(IRepository repo, UriString baseCloneUrl, UriString headCloneUrl, string baseSha, string headSha, string baseRef, string headRef);
 
+        /// <summary>
+        /// Find the merge base SHA between two commits.
+        /// </summary>
+        /// <param name="repository">The repository.</param>
+        /// <param name="targetCloneUrl">The clone url of the PR target repo.</param>
+        /// <param name="baseSha">The PR base SHA.</param>
+        /// <param name="headSha">The PR head SHA.</param>
+        /// <param name="baseRef">The PR base ref (e.g. 'master').</param>
+        /// <param name="pullNumber">The PR number.</param>
+        /// <returns>
+        /// The merge base SHA or null.
+        /// </returns>
+        /// <exception cref="LibGit2Sharp.NotFoundException">Thrown when the merge base can't be found.</exception>
+        Task<string> GetPullRequestMergeBase(IRepository repo, UriString targetCloneUrl, string baseSha, string headSha, string baseRef, int pullNumber);
+
         /// Checks whether the current head is pushed to its remote tracking branch.
         /// </summary>
         /// <param name="repository">The repository.</param>

src/GitHub.InlineReviews/Services/PullRequestSessionService.cs

@@ -122,13 +122,13 @@ public async Task<string> GetPullRequestMergeBase(ILocalRepositoryModel reposito
             }
 
             var repo = await GetRepository(repository);
-            var baseUrl = pullRequest.Base.RepositoryCloneUrl;
+            var targetUrl = pullRequest.Base.RepositoryCloneUrl;
             var headUrl = pullRequest.Head.RepositoryCloneUrl;
             var baseRef = pullRequest.Base.Ref;
-            var headRef = pullRequest.Head.Ref;
+            var pullNumber = pullRequest.Number;
             try
             {
-                mergeBase = await gitClient.GetPullRequestMergeBase(repo, baseUrl, headUrl, baseSha, headSha, baseRef, headRef);
+                mergeBase = await gitClient.GetPullRequestMergeBase(repo, targetUrl, baseSha, headSha, baseRef, pullNumber);
             }
             catch (NotFoundException ex)
             {