testing: Improve git fetch handling to support refspec

For PR testing we need to download quite a different refspec, this patch
provides a better refspec for PR handling.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #744 from kbarber/maint/1.5.x/improve_git_fetch_for_pr_testing

testing: Improve git fetch handling to support refspec