[MERGE] Implement diff --using natively

[Aaron Bentley]

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Aaron Bentley wrote:
> But it might be safe to
> work *best* with tools that emulate gnu diff, and assume exit code 2
> indicates failure, without assuming that 0 or 1 indicate success.

Actually, we can't even do that.  Because exit code 2 can mean "bad
arguments" or it can mean "inputs were binary files".  You have to parse
the output to know which, and that's just not a reasonable way of
handling the general case.

When we start to customize for particular tools, we can add this sort of
detection.  For now, I think we should leave it out.

Aaron
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