git-validation: disable short-subject validation
git-validation: disable short-subject validation

Complains when the commit is a merge commit, it shouldn't!

Discovered in  #2684

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