git-arc: Use --head to avoid changing the checkout.

Rather than using 'git checkout' to move to the commit in question for
create and update, use the '--head' argument to 'arc diff'. This
avoids the need to alter the current checkout and the related bits to
save/restore HEAD in the current checkout.

Reviewed by: imp, markj
Differential Revision: https://reviews.freebsd.org/D36248