[Merge] disable diff3 on win32

[Aaron Bentley]

John Arbash Meinel wrote:
>>>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.

> It seems we must not have used diff3 for applying patches in Arch very
> often. (We used diff to create them, and patch to build revision
> libraries, and revlibs were where corruption was happening).

Right, the only time we used diff3 was in star-merge --threeway (and 
later in baz merge, of course).

> Actually, I think I remember now that I originally introduced a patch
> which added --binary to diff3, and didn't ever fail for me (because it
> was never tested), but someone else ran into the fact that there is no
> --binary flag for diff3.

I think that someone else may even have been me.

> Anyway, the other point I would want to mention is that probably diff3
> isn't safe under cygwin either. Though I suppose it is more likely to be
> safe than win32.

With cygwin, the user gets to choose whether or not to mangle line 
endings, so if they do, at least it's the user's decision.

Aaron