Fixed possible null reference exception, fixes #1022

Author: JakeGinnivan

src/GitVersionCore/GitVersionCacheKeyFactory.cs

@@ -79,7 +79,7 @@ private static List<string> CalculateDirectoryContents(string root)
                     continue;
                 }
 
-                string[] files = null;
+                string[] files;
                 try
                 {
                     files = Directory.GetFiles(currentDir);

src/GitVersionCore/LibGitExtensions.cs

@@ -68,7 +68,7 @@ public static Commit FindCommitBranchWasBranchedFrom([NotNull] this Branch branc
                         mergeBaseCommit = findMergeBase,
                         branch = otherBranch
                     };
-                }).Where(b => b != null).OrderByDescending(b => b.mergeBaseCommit.Committer.When).ToList();
+                }).Where(b => b.mergeBaseCommit != null).OrderByDescending(b => b.mergeBaseCommit.Committer.When).ToList();
 
                 var firstOrDefault = mergeBases.FirstOrDefault();
                 if (firstOrDefault != null)