Make baselines faster by not writing out unneeded files

[RyanCavanaugh]

Two changes here

The first is to not write out baseline files to /local when the content is already identical to what's in /reference. Instead, we write out new files when they've changed, and write .delete files when the file exists in /reference but not in /local

The second is to update the baseline-accept tasks to simply copy with overwrite, plus treat .delete files specially so they delete the destination file. These are now basically "soft by default" so you can run arbitrary subsets of tests and baseline-accept without having to worry about nuking your reference folder.

Locally, these brought the total time to run the 'conformance' suite from 1m33s to 0m54s. Might even be a bigger gain on non-SSD machines (not sure what travis is using).

[RyanCavanaugh]

🔔

[weswigham]

👍 if you're satisfied with it. I don't see any issues. We could probably do even better if we just wrote all the files-to-delete out to a single manifest file all at once (rather than a smattering of .delete files), but that needn't be considered in this PR.

[mhegazy]

👍

[HerringtonDarkholme]

I have run jake rutests locally and cannot get diff by jake diff. It seems jake will always remove local baseline and no diff can be produced.