[Merge] disable diff3 on win32

[Aaron Bentley]

John Arbash Meinel wrote:
> Aaron Bentley wrote:
> 
>>This patch disables diff3 on win32, because typical diff3
>>implementations on win32 do not properly preserve line endings.

> Doesn't diff3 support the --binary flag (same as diff & patch). I
> remember dealing with all of this for Arch.

No, it doesn't.  At least, 2.8.7 doesn't.

> I realize there is no "native" diff3 on Windows anyway. You have to
> install it in some non-standard method. But I don't think you have to
> specifically disable the tests.

We have to specifically disable diff3 because it will mangle 
line-endings, and I don't want that happening to users.  If we disable 
it without disabling the tests, we'll get test case failures.

Aaron