accommodate git's diff.suppress-blank-empty=true setting
accommodate git's diff.suppress-blank-empty=true setting

Summary:
accommodate git's diff.suppress-blank-empty=true setting
Without this change, if you were to set diff.suppress-blank-empty=true
in your .gitconfig (as I do), then "arc diff" would always fail with the
cryptic diagnostic, "Diff Parse Exception: Found the wrong number of
hunk lines."

Test Plan:
Put this in ~/.gitconfig or .git/config
[diff]
         suppress-blank-empty = true
and run "arc lint".  It should pass.  Without this chnage,
it would fail as described above.

Reviewers: vrana, epriestley

Reviewed By: vrana

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3963

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