use git diff relative for onlyChanges
use git diff relative for onlyChanges

Summary:
	- in case testPathDirs and git root are different, path will be relative to root
	- fixes https://github.com/facebook/jest/issues/327
Closes https://github.com/facebook/jest/pull/674

Reviewed By: svcscm

Differential Revision: D2840629

fb-gh-sync-id: cb3be89a45742551d4042080bfa065fba04c7110

facebook/jest,Daniel15/jest,yungsters/jest,blainekasten/jest,skovhus/jest,razh/jest,mpontus/jest,ide/jest,sampepose/jest,yungsters/jest,aaron-goshine/jest,Daniel15/jest,blainekasten/jest,ide/jest,mpontus/jest,ide/jest,facebook/jest,cpojer/jest,razh/jest,ncuillery/jest,aaron-goshine/jest,skawian/jest,skovhus/jest,bookman25/jest,skawian/jest,cpojer/jest,bookman25/jest,alvinsj/jest,ncuillery/jest,bookman25/jest,facebook/jest,blainekasten/jest,skovhus/jest,mpontus/jest,Daniel15/jest,sampepose/jest,aaron-goshine/jest,alvinsj/jest,cpojer/jest