Don't use absolute path in `ArcanistDiffUtils`
Don't use absolute path in `ArcanistDiffUtils`

Summary: Improves Windows compatibility.

Test Plan: Ran failing unit test on Windows.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5344

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