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