Explicitly said 'LibGet2Sharp.Index' to avoid confusion with System.Index (build failed on Linux)

Author: erikbra

src/GitVersionCore.Tests/Mocks/MockRepository.cs

@@ -211,7 +211,7 @@ public void RevParse(string revision, out Reference reference, out GitObject obj
 
         public Branch Head { get; set; }
         public LibGit2Sharp.Configuration Config { get; set; }
-        public Index Index { get; set; }
+        public LibGit2Sharp.Index Index { get; set; }
         public ReferenceCollection Refs { get; set; }
 
         public IQueryableCommitLog Commits