Work with ConvertToUTF8 plugin.

ConvertToUTF8 store the file's encoding in the setting "origin_encoding". For details, please see: https://github.com/seanliang/ConvertToUTF8/blob/master/ConvertToUTF8.py

Without this patch, GitGutter will fail to detect right encoding for a GBK content file opened with ConvertToUTF8. 

ConvertToUTF8 is a very popular plugin (Top 25, see https://sublime.wbond.net/packages/ConvertToUTF8), especially among Chinese. Of course it's up for you to decide whether this patch is worthy.