Summary:
The sandcastle setup doesn't provide a remote with our branches. Need
to fetch them directly from github.
Test Plan:
ran script on devserver and the relevant commands on a sandcastle
host.
Reviewers: IslamAbdelRahman, lightmark, kradhakrishnan, sdong
Reviewed By: sdong
Subscribers: andrewkr, dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D59511