GEODE-6059: PR pipeline gets merge-base from resource (#2877)
GEODE-6059: PR pipeline gets merge-base from resource (#2877)

Keeps the PR from needing the entire repo for faster testing

Authored-by: Robert Houghton <8ee5723e9b22f5849055c1010807733cae5c117f@pivotal.io>
davinash/geode,smgoller/geode,davinash/geode,jdeppe-pivotal/geode,masaki-yamakawa/geode,davebarnes97/geode,masaki-yamakawa/geode,davinash/geode,jdeppe-pivotal/geode,masaki-yamakawa/geode,masaki-yamakawa/geode,smgoller/geode,jdeppe-pivotal/geode,davebarnes97/geode,smgoller/geode,PurelyApplied/geode,davebarnes97/geode,smgoller/geode,davebarnes97/geode,jdeppe-pivotal/geode,davinash/geode,PurelyApplied/geode,davinash/geode,PurelyApplied/geode,davebarnes97/geode,jdeppe-pivotal/geode,PurelyApplied/geode,PurelyApplied/geode,PurelyApplied/geode,PurelyApplied/geode,davinash/geode,smgoller/geode,jdeppe-pivotal/geode,masaki-yamakawa/geode,davebarnes97/geode,davinash/geode,masaki-yamakawa/geode,masaki-yamakawa/geode,davebarnes97/geode,smgoller/geode,jdeppe-pivotal/geode,smgoller/geode