No output on stderr when using is_git() in case it fails
No output on stderr when using is_git() in case it fails

Summary:
When using is_git() on a folder that's not a git repository, it output an error on stderr.
We don't want that.
The diff fixes it.

Test Plan: sandcastle

Reviewed By: bertmaher

fbshipit-source-id: 1101e45

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