[xaprepare] Add a missing line to fix for issue 5867 (#5875)

Context: #5867
Context: 40fdecb

Assign new array to the old one so that `IndexOutOfRangeException`
isn't thrown later on in the code.

build-tools/xaprepare/xaprepare/ToolRunners/GitRunner.BlamePorcelainEntry.cs

@@ -72,6 +72,7 @@ void StoreHeader (string line)
 					}
 
 					var newParts = new string[] { parts [0], String.Empty };
+					parts = newParts;
 					Log.Instance.Warning ($"Unexpected commit header format (wrong number of fields): {line}");
 					Log.Instance.Warning ("Using empty string for the header value");
 				}