Make formatting test independent on pre-existing changes in git
Make formatting test independent on pre-existing changes in git

Summary:
The formatting test uses git index to check if there are some changes required. This doesn't work when there are changes in git index before this job.

The solution is to stash changes (including untracked files) and restore the stash after the test.

Test Plan: Travis

Reviewed By: ttsugriy

fbshipit-source-id: 8db8c36

LegNeato/buck,nguyentruongtho/buck,robbertvanginkel/buck,brettwooldridge/buck,robbertvanginkel/buck,shs96c/buck,dsyang/buck,JoelMarcey/buck,brettwooldridge/buck,zpao/buck,romanoid/buck,nguyentruongtho/buck,k21/buck,shs96c/buck,LegNeato/buck,clonetwin26/buck,SeleniumHQ/buck,shs96c/buck,zpao/buck,LegNeato/buck,shs96c/buck,Addepar/buck,robbertvanginkel/buck,brettwooldridge/buck,romanoid/buck,clonetwin26/buck,zhan-xiong/buck,romanoid/buck,zhan-xiong/buck,brettwooldridge/buck,shs96c/buck,rmaz/buck,dsyang/buck,facebook/buck,brettwooldridge/buck,romanoid/buck,k21/buck,JoelMarcey/buck,kageiit/buck,k21/buck,rmaz/buck,k21/buck,robbertvanginkel/buck,clonetwin26/buck,nguyentruongtho/buck,ilya-klyuchnikov/buck,ilya-klyuchnikov/buck,ilya-klyuchnikov/buck,shs96c/buck,clonetwin26/buck,ilya-klyuchnikov/buck,JoelMarcey/buck,rmaz/buck,k21/buck,zhan-xiong/buck,JoelMarcey/buck,robbertvanginkel/buck,JoelMarcey/buck,clonetwin26/buck,SeleniumHQ/buck,clonetwin26/buck,shybovycha/buck,brettwooldridge/buck,clonetwin26/buck,kageiit/buck,JoelMarcey/buck,Addepar/buck,zhan-xiong/buck,LegNeato/buck,clonetwin26/buck,JoelMarcey/buck,LegNeato/buck,clonetwin26/buck,zpao/buck,zhan-xiong/buck,facebook/buck,k21/buck,facebook/buck,SeleniumHQ/buck,rmaz/buck,dsyang/buck,kageiit/buck,rmaz/buck,robbertvanginkel/buck,JoelMarcey/buck,robbertvanginkel/buck,SeleniumHQ/buck,SeleniumHQ/buck,facebook/buck,robbertvanginkel/buck,facebook/buck,romanoid/buck,rmaz/buck,zhan-xiong/buck,LegNeato/buck,shybovycha/buck,shybovycha/buck,ilya-klyuchnikov/buck,shs96c/buck,rmaz/buck,LegNeato/buck,shs96c/buck,JoelMarcey/buck,brettwooldridge/buck,LegNeato/buck,ilya-klyuchnikov/buck,ilya-klyuchnikov/buck,JoelMarcey/buck,shs96c/buck,LegNeato/buck,dsyang/buck,nguyentruongtho/buck,shs96c/buck,dsyang/buck,zpao/buck,dsyang/buck,zpao/buck,romanoid/buck,zhan-xiong/buck,SeleniumHQ/buck,shybovycha/buck,zpao/buck,robbertvanginkel/buck,shs96c/buck,shybovycha/buck,romanoid/buck,shybovycha/buck,Addepar/buck,Addepar/buck,rmaz/buck,SeleniumHQ/buck,LegNeato/buck,nguyentruongtho/buck,SeleniumHQ/buck,zhan-xiong/buck,zhan-xiong/buck,clonetwin26/buck,dsyang/buck,Addepar/buck,k21/buck,shybovycha/buck,kageiit/buck,rmaz/buck,robbertvanginkel/buck,LegNeato/buck,zpao/buck,clonetwin26/buck,zhan-xiong/buck,Addepar/buck,brettwooldridge/buck,brettwooldridge/buck,Addepar/buck,k21/buck,romanoid/buck,shybovycha/buck,brettwooldridge/buck,k21/buck,nguyentruongtho/buck,shybovycha/buck,LegNeato/buck,SeleniumHQ/buck,clonetwin26/buck,ilya-klyuchnikov/buck,zhan-xiong/buck,facebook/buck,Addepar/buck,Addepar/buck,JoelMarcey/buck,Addepar/buck,LegNeato/buck,kageiit/buck,dsyang/buck,dsyang/buck,facebook/buck,JoelMarcey/buck,k21/buck,kageiit/buck,rmaz/buck,dsyang/buck,brettwooldridge/buck,romanoid/buck,ilya-klyuchnikov/buck,robbertvanginkel/buck,brettwooldridge/buck,nguyentruongtho/buck,k21/buck,dsyang/buck,zhan-xiong/buck,SeleniumHQ/buck,shybovycha/buck,SeleniumHQ/buck,ilya-klyuchnikov/buck,shybovycha/buck,Addepar/buck,robbertvanginkel/buck,romanoid/buck,rmaz/buck,shs96c/buck,rmaz/buck,robbertvanginkel/buck,shybovycha/buck,zhan-xiong/buck,romanoid/buck,shs96c/buck,dsyang/buck,clonetwin26/buck,SeleniumHQ/buck,romanoid/buck,romanoid/buck,dsyang/buck,rmaz/buck,k21/buck,Addepar/buck,SeleniumHQ/buck,ilya-klyuchnikov/buck,ilya-klyuchnikov/buck,shybovycha/buck,k21/buck,Addepar/buck,JoelMarcey/buck,kageiit/buck,brettwooldridge/buck,ilya-klyuchnikov/buck