Allow users naming their repositories differently

Summary:
If someone names their repository something other than
settings.SANDCASTLE_REPO, we fail to fetch from it because we assume
that all remote repositories are named this. This change pulls their
repository name from the pull request information. This isn't even close
to perfect (for example, if someone tries to request a single branch
separately, it will break everything for them) but it should fix a
couple weird cases.
(c.f. https://github.com/Khan/khan-exercises/pull/40939)

Test Plan:
Reset the repository by getting rid of all remotes. (because we
cache these :( ) Go to /pull/40939, make sure that it works.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2065