encoding error with gitfs and files with different encoding from global one

Description of Issue

with salt 3000 on python 3 and a gitfs source, if you try to use a path from git containing a text file in an encoding different from the system one, but that contains valid utf8 (like ascii) in its first 2k, it fails with an error similar to this one:

2020-04-03 19:53:22,663 [salt.master      :1846][ERROR   ][61192] Error in function _serve_file:
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/salt/master.py", line 1839, in run_func
    ret = getattr(self, func)(load)
  File "/usr/lib/python3.6/site-packages/salt/fileserver/__init__.py", line 650, in serve_file
    return self.servers[fstr](load, fnd)
  File "/usr/lib/python3.6/site-packages/salt/fileserver/gitfs.py", line 170, in serve_file
    return _gitfs().serve_file(load, fnd)
  File "/usr/lib/python3.6/site-packages/salt/utils/gitfs.py", line 2861, in serve_file
    data = data.decode(__salt_system_encoding__)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe8 in position 4193: invalid continuation byte

Versions Report

Salt Version:
           Salt: 3000.1
 
Dependency Versions:
           cffi: Not Installed
       cherrypy: unknown
       dateutil: 2.8.1
      docker-py: Not Installed
          gitdb: 2.0.6
      gitpython: 3.0.5
         Jinja2: 2.10.3
        libgit2: Not Installed
       M2Crypto: 0.33.0
           Mako: 1.1.0
   msgpack-pure: Not Installed
 msgpack-python: 0.6.2
   mysql-python: 1.4.5
      pycparser: Not Installed
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 3.6.8 (default, Aug  7 2019, 17:28:10)
   python-gnupg: Not Installed
         PyYAML: 3.12
          PyZMQ: 15.3.0
          smmap: 2.0.5
        timelib: Not Installed
        Tornado: 4.5.3
            ZMQ: 4.1.4
 
System Versions:
           dist: centos 7.7.1908 Core
         locale: UTF-8
        machine: x86_64
        release: 3.10.0-1062.4.3.el7.x86_64
         system: Linux
        version: CentOS Linux 7.7.1908 Core

Reproduce the failure with a gitfs text file whose encoding differs from the system encoding but begins with valid UTF-8. Start at salt/utils/gitfs.py in serve_file(), around the reported decode at line 2861, and trace how the file data is decoded. Done means gitfs serves the file without the UnicodeDecodeError.