Add GH pull request refspec to install_from_git

 - When testing out Jenkins job configuration tweaks that must coincide with source
   changes, it can be useful to have Beaker be aware of the GitHub refspec.  This
   allows SHA references to GitHub commits / PRs that have not yet been merged, 
   without having to jump through a number of additional job configurations to gain
   access to the appropriate commits.
 - Note that this does have performance implications, as performing a fetch will take
   longer on the first time.