Uninitialised variable in C++ method diff_xIndex

```
I ran my application through valgrind and it exposed a case where
lastDiff.operation was used without being initialised within diff_xIndex. It
may never be assigned to in the loop and so the test against DELETE is against
an uninitialised value since the Diff default constructor does not set a value.

Either the default constructor should set a value or the code should be altered
to mimic the Java version with lastDiff being a pointer to a Diff.

```

Original issue reported on code.google.com by `fotherin...@gmail.com` on 18 Feb 2012 at 9:47