Examine upstream path instead of assuming "origin"

Summary:
Instead of blindly assuming that "origin" is the repository that
arcanist should communicate with, use the remote that is configured
for the branch in git.

Test Plan:
Used arc which with a branch with no upstream, an
origin/master upstream, and an upstream/master upstream -- the last of
which is being used to create and land this diff.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: joshuaspence, Korvin

Differential Revision: https://secure.phabricator.com/D14530