[PATCH 1 of 2] extdiff: support tools that can be run simultaneously

[Ludovic Chabant]

> Generally looks good. Can you add some tests?

Oh right, that's another thing I wanted to ask -- how would I test that? The only idea I have is to log some verbose message ("tool %s has a graphical interface, launching processes simultaneously") and detect that in test. Is there any better way?

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