gitfs/git_pillar: Fix UnicodeDecodeError while cleaning stale refs

[sathieu]

What does this PR do?

It fixes:

[salt.utils.gitfs :2341][ERROR   ][866] Exception caught while fetching gitfs remote '__env__ https://git.example.org/somerepo.git': 'ascii' codec can't decode byte 0xc3 in position 0: ordinal not in range(128)

The environement variables are not sanitized, and as we use LANG=fr_FR.utf8. Calling the same command prints non-ascii:

root@salt-master:/var/cache/salt/master/git_pillar/51958f355141ebce331e186a5230c20f2d0264025054eeb2fc2c5b3b81ab53a0# git remote prune origin
Élimination de origin
URL : https://git.example.org/somerepo.git
* [éliminé] origin/toto

Inspired by https://github.com/gitpython-developers/GitPython/blob/a8591a094a768d73e6efb5a698f74d354c989291/git/cmd.py#L694

What issues does this PR fix or reference?

None

Previous Behavior

An exception is raised (and the flow is stopped, but will be successful on the next try as the remote is now pruned):

2018-12-06 11:57:11,495 [salt.utils.gitfs :2341][ERROR   ][866] Exception caught while fetching gitfs remote '__env__ https://git.example.org/somerepo.git': 'ascii' codec can't decode byte 0xc3 in position 0: ordinal not in range(128)
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/salt/utils/gitfs.py", line 2329, in fetch_remotes
    if repo.fetch():
  File "/usr/lib/python2.7/dist-packages/salt/utils/gitfs.py", line 757, in fetch
    return self._fetch()
  File "/usr/lib/python2.7/dist-packages/salt/utils/gitfs.py", line 1271, in _fetch
    cleaned = self.clean_stale_refs()
  File "/usr/lib/python2.7/dist-packages/salt/utils/gitfs.py", line 605, in clean_stale_refs
    if line.startswith(marker):
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 0: ordinal not in range(128)

New Behavior

No more exception. And the fetch succesfuly terminates.

Tests written?

No

Commits signed with GPG?

Yes