[MERGE] Handle missing tools gracefully for diff --using (was: [MERGE] Implement diff --using natively)

[John Arbash Meinel]

John Arbash Meinel has voted approve.
Status is now: Superseded
Comment:
why is this considered "superseded"?

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C477526FA.4050006%40aaronbentley.com%3E