Convert byte strings to utf-8 string when communicating with git in buck_version.py
Convert byte strings to utf-8 string when communicating with git in buck_version.py

Summary:
Using byte strings can lead to problems like in https://github.com/facebook/buck/issues/1812

I discovered this problem when trying to build Buck on Windows.

Reviewed By: bobyangyf

fbshipit-source-id: 8994c2f013

rmaz/buck,JoelMarcey/buck,nguyentruongtho/buck,romanoid/buck,kageiit/buck,JoelMarcey/buck,rmaz/buck,facebook/buck,facebook/buck,romanoid/buck,nguyentruongtho/buck,brettwooldridge/buck,brettwooldridge/buck,kageiit/buck,rmaz/buck,rmaz/buck,JoelMarcey/buck,JoelMarcey/buck,Addepar/buck,brettwooldridge/buck,SeleniumHQ/buck,brettwooldridge/buck,SeleniumHQ/buck,JoelMarcey/buck,nguyentruongtho/buck,romanoid/buck,kageiit/buck,Addepar/buck,SeleniumHQ/buck,Addepar/buck,Addepar/buck,rmaz/buck,brettwooldridge/buck,romanoid/buck,romanoid/buck,facebook/buck,JoelMarcey/buck,rmaz/buck,romanoid/buck,rmaz/buck,zpao/buck,Addepar/buck,zpao/buck,facebook/buck,JoelMarcey/buck,zpao/buck,Addepar/buck,SeleniumHQ/buck,brettwooldridge/buck,romanoid/buck,brettwooldridge/buck,romanoid/buck,brettwooldridge/buck,Addepar/buck,Addepar/buck,nguyentruongtho/buck,JoelMarcey/buck,facebook/buck,rmaz/buck,SeleniumHQ/buck,JoelMarcey/buck,romanoid/buck,JoelMarcey/buck,zpao/buck,zpao/buck,JoelMarcey/buck,kageiit/buck,nguyentruongtho/buck,kageiit/buck,Addepar/buck,facebook/buck,rmaz/buck,facebook/buck,Addepar/buck,zpao/buck,SeleniumHQ/buck,rmaz/buck,Addepar/buck,nguyentruongtho/buck,rmaz/buck,Addepar/buck,kageiit/buck,brettwooldridge/buck,brettwooldridge/buck,romanoid/buck,nguyentruongtho/buck,zpao/buck,SeleniumHQ/buck,Addepar/buck,JoelMarcey/buck,romanoid/buck,SeleniumHQ/buck,brettwooldridge/buck,rmaz/buck,brettwooldridge/buck,SeleniumHQ/buck,rmaz/buck,SeleniumHQ/buck,kageiit/buck,SeleniumHQ/buck,romanoid/buck,SeleniumHQ/buck,SeleniumHQ/buck,JoelMarcey/buck,romanoid/buck,brettwooldridge/buck