Provide a git commit template.
Provide a git commit template.

Summary: adds a Differential-suitable git commit template.

Test Plan: meta

Reviewers:

CC:

dguo-coursera/arcanist,julkiewicz/arcanist,jra3/arcanist,julkiewicz/arcanist,fivestars/arcanist,calblueprint/arcanist,hach-que/arcanist,phacility/arcanist,folsom-labs/arcanist,memsql/arcanist,r4nt/arcanist,zhihu/arcanist,wikimedia/phabricator-arcanist,wharris/arcanist,wharris/arcanist,alexmv/arcanist,zhihu/arcanist,Khan/arcanist,memsql/arcanist,fivestars/arcanist,eSpark/arcanist,hach-que/arcanist,folsom-labs/arcanist,r4nt/arcanist,phacility/arcanist,wxstars/arcanist,codelabs-ch/arcanist,aik099/arcanist,kevinseelbach/arcanist,alexmv/arcanist,chundongwang/arcanist,haskell-infra/arcanist,aik099/arcanist,Symplicity/arcanist,haskell-infra/arcanist,PageUpPeopleOrg/arcanist-patches,memsql/arcanist,eSpark/arcanist,codelabs-ch/arcanist,jra3/arcanist,Symplicity/arcanist,haskell-infra/arcanist,r4nt/arcanist,wikimedia/phabricator-arcanist,Symplicity/arcanist,coursera/arcanist,chundongwang/arcanist,calblueprint/arcanist,Khan/arcanist,zhihu/arcanist,wxstars/arcanist,codelabs-ch/arcanist,sneakpeeq/arcanist,phacility/arcanist,coursera/arcanist,KieranHayes/arcanist,memsql/arcanist,folsom-labs/arcanist,julkiewicz/arcanist,coursera/arcanist,hach-que/arcanist,folsom-labs/arcanist,eSpark/arcanist,phacility/arcanist,sneakpeeq/arcanist,alexmv/arcanist,sneakpeeq/arcanist,kevinseelbach/arcanist,wikimedia/phabricator-arcanist,KieranHayes/arcanist,uber/arcanist,haskell-infra/arcanist,alexmv/arcanist,phacility/arcanist,r4nt/arcanist,calblueprint/arcanist,dguo-coursera/arcanist,wxstars/arcanist,codelabs-ch/arcanist,KieranHayes/arcanist,jra3/arcanist,sneakpeeq/arcanist,PageUpPeopleOrg/arcanist-patches,fivestars/arcanist,Khan/arcanist,zhihu/arcanist,kevinseelbach/arcanist,PageUpPeopleOrg/arcanist-patches,wikimedia/phabricator-arcanist,hach-que/arcanist,dguo-coursera/arcanist,eSpark/arcanist,codelabs-ch/arcanist,uber/arcanist,wharris/arcanist,calblueprint/arcanist,uber/arcanist,wxstars/arcanist,coursera/arcanist,uber/arcanist,aik099/arcanist,fivestars/arcanist,kevinseelbach/arcanist,Symplicity/arcanist,KieranHayes/arcanist,chundongwang/arcanist,julkiewicz/arcanist,chundongwang/arcanist,PageUpPeopleOrg/arcanist-patches,aik099/arcanist,jra3/arcanist,Khan/arcanist,wharris/arcanist,dguo-coursera/arcanist