add short option for fetching git revision hash
add short option for fetching git revision hash

Summary:
Sometimes we need to show abbreviated hashes, passing --short is more
reliable than substring of fixed number of characters.

Test Plan:
- called with and without the parameter, got correct results
           -

Reviewed By: jungejason
Reviewers: jungejason
CC: jungejason
Revert Plan:
sure

Other Notes:

Differential Revision: 170

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