tests: find LCA to detect commit differences
tests: find LCA to detect commit differences

Creates a function to find the lowest common ancestor between two commits.

Fixes #2423

intelsdi-x/rkt,coreos/rkt,mike1729/rkt,tmrts/rkt,dgonyeo/rkt,nhlfr/rkt,sstabellini/rkt,ybubnov/rkt,akshaykarle/rkt,monstermunchkin/rkt,Wilybald/rkt,shobhit85/rkt,joshix/rkt,cab105/rkt,jjlakis/rkt,achanda/rkt,sgotti/rkt,coreos/rkt,cab105/rkt,jonboulle/rkt,s-urbaniak/rkt,sgotti/rkt,mike1729/rkt,matthaias/rkt,rkt/rkt,Wilybald/rkt,coreos/rkt,lucasallan/rkt,squeed/rkt,jakon89/rkt,kinvolk/rkt,yifan-gu/rocket,jonboulle/rkt,tmrts/rkt,sstabellini/rkt,euank/rkt,jonboulle/rkt,shobhit85/rkt,intelsdi-x/rkt,kinvolk/rkt,rkt/rkt,intelsdi-x/rkt,tmrts/rkt,lucasallan/rkt,ybubnov/rkt,alban/rkt,dgonyeo/rkt,kinvolk/rkt,monstermunchkin/rkt,Shashwatsh/rkt,s-urbaniak/rkt,sstabellini/rkt,jjlakis/rkt,monstermunchkin/rkt,markllama/rkt,akshaykarle/rkt,markllama/rkt,lucasallan/rkt,Shashwatsh/rkt,shobhit85/rkt,yifan-gu/rocket,rkt/rkt,Wilybald/rkt,joshix/rkt,squeed/rkt,alban/rkt,yifan-gu/rkt,sgotti/rkt,jakon89/rkt,matthaias/rkt,dgonyeo/rkt,yifan-gu/rkt,akshaykarle/rkt,rkt/rkt,yifan-gu/rkt,cab105/rkt,matthaias/rkt,euank/rkt,kinvolk/rkt,joshix/rkt,markllama/rkt,achanda/rkt,alban/rkt,joshix/rkt,achanda/rkt,nhlfr/rkt,jjlakis/rkt,jakon89/rkt,yifan-gu/rocket,Wilybald/rkt,ybubnov/rkt,euank/rkt,mike1729/rkt,Shashwatsh/rkt,squeed/rkt,s-urbaniak/rkt,lucasallan/rkt,nhlfr/rkt