[PATCH V3] extdiff: add --per-file and --confirm options

[Ludovic Chabant]

> Can you split this to a separate patch? It'll need a bit more churn,
> and we wouldn't want to make it a blocker of the --per-file/--confirm
> feature.

I just sent PATCH V4 which should be just that feature alone, where, if
you don't specify --confirm, it launches the external tool one by one.

The second patch should be ready soon.

Thanks!
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