fix(remotes): Tweak fetching to hopefully be more robust.

At the moment builtbot keeps having trouble fetching changes from
GitHub. It seems possible that multiple jobs each using 8 threads to
fetch code via HTTPS is triggering some sort of DOS protection, causing
many of our requests to fail. To avoid this here are a few changes taken
from the CyanogenMod project which also uses GitHub:

 - Reduce threads to 4 (sync-j)
 - Request only the branches specified by the manifest (sync-c)
 - Use a relative path for the github remote. This means that all other
   projects will be fetched using the same protocol://hostname that the
   manifest was cloned over. That way we aren't stuck with HTTPS only.
 - For funzies add a private GitHub remote that uses ssh. This can be
   useful for adding/overriding projects in local_manifest.xml

Hopefully this will help keep our buildbot and GitHub on speaking terms.

Change-Id: I77cfa2d9e9468f5d39fce0d35dbfc45fb6f94967