[arcconfig] Default base to previous revision

When you run `arc diff`, arc defaults to uploading all the changes you
have against the upstream branch into a single patch. This is almost
never what you want for stacked commits (patch series); you only want to
submit the changes done by the current patch. It's also come up as a
point of confusion in the Phabricator vs. GitHub PRs discussion, for
example. Configure arc to only upload your current patch by default,
which I think is a much more suitable default for LLVM developers.

Differential Revision: https://reviews.llvm.org/D74990