feat(broccoli): allow rebuild() to return DiffResult

[caitp]

Plugins may opt to return a DiffResult themselves, and avoid the
need to calculate a diff

[caitp]

@IgorMinar I've experimented with converting plugins to return DiffResults, but have had (once again) issues with dead symlinks, relating to either MultiCopy or some of the Stew plugins. So it might need some work.

The performance for subsequent builds does seem to be very good when the build works correctly though (with just a handful of plugins updated) ~on the order of 25ms per tree

[IgorMinar]

returning a promise for a diffresult should be possible too. no?

[caitp]

It is, and there's code handling that case here --- but it seems weird to have Promise<any> | Promise<DiffResult>, since any is kind of catch-all

[IgorMinar]

how about Promise<undefined|DiffResult>?

[caitp]

done

[caitp]

I think that addresses each comment

[IgorMinar]

Please add a comment that this.treeDiffer attempts to reads the diff from the input tree where we stash after the rebuild of that tree is done.

[IgorMinar]

otherwise this looks good

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