Fetch PR head before base

This avoids two fetches in the simple case where head is downstream of base.

Author: jcansdale

src/GitHub.App/Services/GitClient.cs

@@ -386,17 +386,6 @@ public async Task<string> GetPullRequestMergeBase(IRepository 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)
             {
@@ -409,6 +398,17 @@ public async Task<string> GetPullRequestMergeBase(IRepository repo,
                 }
             }
 
+            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 mergeBaseCommit = repo.ObjectDatabase.FindMergeBase(baseCommit, headCommit);
             if (mergeBaseCommit == null)
             {