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

.arcconfig

@@ -1,4 +1,6 @@
 {
   "project_id" : "peru",
-  "conduit_uri" : "https://phabricator.buildinspace.com"
+  "conduit_uri" : "https://phabricator.buildinspace.com",
+  "arc.feature.start.default" : "origin/master",
+  "git.default-relative-commit": "HEAD^"
 }