Fix tab indentation missing in Diffusion

This seems to be a specific of how browsers are dealing with
spaces/tabs. Multiple spaces works just fine, but multiple
tabs were treating as a single space which breaks indentation.

Now made it so tabs are replaced with 4 spaces. Not ideal but
still better than fully unreadable code. This also matches to
how differential is handling tabs.

Ref T2495. See: <https://github.com/facebook/phabricator/issues/487>

Reviewed by: epriestley