Previously it was possible for the DiffChunkGenerator to update the

line counts in a forced interdiff (i.e., an interdiff where there is no

corresponding interfilediff, such as when the changes to a file are

reverted). This results in all insert and delete counts being swapped

in the FileDiff's line counts. This patch fixes this issue.
A unit test was added to ensure that this does not regress.

Ran unit tests.

The new unit test failed without the patch applied.