Added support for passing a commit id from master branch for merge
Added support for passing a commit id from master branch for merge

jorrit-steporange/CumulusCI,steelbrick/CumulusCI,Joble/CumulusCI,motivislearning/CumulusCI,ddrezga/CumulusCI,jorrit-steporange/CumulusCI,e02d96ec16/CumulusCI,SalesforceFoundation/CumulusCI,ddrezga/CumulusCI,roundcorner-products/CumulusCI,steelbrick/CumulusCI,roundcorner-products/CumulusCI,SalesforceFoundation/CumulusCI,e02d96ec16/CumulusCI,cdcarter/CumulusCI,motivislearning/CumulusCI