[PR] Ignore opam switch folder
[PR] Ignore opam switch folder

Summary:
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This ignores `_opam` folder
Pull Request resolved: https://github.com/facebook/flow/pull/7748

Differential Revision: D15698238

Pulled By: mroch

fbshipit-source-id: 7b8a4aa1f5642febb82c96ac433cfb27f9cb3b83

gabelevi/flow,mroch/flow,gabelevi/flow,mroch/flow,facebook/flow,samwgoldman/flow,samwgoldman/flow,facebook/flow,facebook/flow,samwgoldman/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,samwgoldman/flow,samwgoldman/flow,samwgoldman/flow,gabelevi/flow,gabelevi/flow,mroch/flow,gabelevi/flow,samwgoldman/flow,mroch/flow,gabelevi/flow,gabelevi/flow