Simplify build file from data-or-hash code

Summary:
We have a bit more copy-paste than we need, consolidate a bit.

(Also switch Mercurial to download git diffs, which it handles well; we use them in "arc patch".)

Test Plan:

• Downloaded a raw diff from Differential.
• Downloaded a raw change from Diffusion.
• Downloaded a raw file from Diffusion.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2942