Dpl: Allow github pages deployments to retain history

[jyasskin]

The W3C deployment scripts that I'm trying to replace retain the history of the gh-pages branch.

If this is a change you'd accept, would you rather an option to retain history, or just unconditionally do it?

[BanzaiMan]

If retaining history is possible, then we should.

[jyasskin]

It's totally possible. I assume you mean we should do it unconditionally.

The change will conflict with #695, so I'd appreciate you merging (or reviewing) that one, and I'll work on this one on top of it when I get back from vacation next week.

[polyzen]

See also travis-ci/travis-ci/issues/7562