configure arc to diff against HEAD^ and create branches against origin/master

Summary:
By default, `arc diff` includes all the changes between the current branch and
origin/master. I prefer to do my own squashing, and often my diffs are on top
of each other, so `HEAD^` works much better for me. (It's also what we use in
fb.)

Also by default, `arc feature` creates new branches off of the current head,
just like `git branch` does. Since it's just a convenience feature, I'd prefer
to have it do the thing that's most convenient for me, and branch off of
origin/master instead. When I need something more complicated, I'm not going to
be using this wrapper anyway.

@sean what are your thoughts on these configs?

Reviewers: sean

Reviewed By: sean

Subscribers: sean

Differential Revision: https://phabricator.buildinspace.com/D7