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

[Ludovic Chabant]

> > 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?
> 
> Something like that. Maybe you can use 'sleep x; echo y' (x/y depending on e.g.
> filename) as a merge tool to check if the tool spawns asynchronously or not,
> if that improves the test coverage.

I opted for the verbose trace -- I figured I didn't want to contribute to making the test suite run noticeably longer, and the tricky code to test is figuring out if we should spawn processes asynchronously or not -- not the code that actually does the spawning.

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