Fix `arc unit` when running from non-root directory
Fix `arc unit` when running from non-root directory

Summary:
`$this->getPaths()` gives us paths relative to repository root.
We resolve them relative to cwd.

Test Plan: [src] `arc unit difference`

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2634

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