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

[Yuya Nishihara]

On Sun, 03 Feb 2019 08:52:47 -0800, Ludovic Chabant wrote:
> > 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?

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.