git-prompt: fix LANG override for git status call

The previous version modified the whole environment leading to
problems (see #7757). This version *adds* LANG=C to the current
env, without overriding it completely.

Fixes #7757.

plugins/git-prompt/gitstatus.py

 #!/usr/bin/env python
 from __future__ import print_function
 
+import os
 import sys
 import re
 import shlex
@@ -30,7 +31,7 @@ def get_tagname_or_hash():
 
 # `git status --porcelain --branch` can collect all information
 # branch, remote_branch, untracked, staged, changed, conflicts, ahead, behind
-po = Popen(['git', 'status', '--porcelain', '--branch'], env={"LANG": "C"}, stdout=PIPE, stderr=PIPE)
+po = Popen(['git', 'status', '--porcelain', '--branch'], env=dict(os.environ, LANG="C"), stdout=PIPE, stderr=PIPE)
 stdout, sterr = po.communicate()
 if po.returncode != 0:
     sys.exit(0)  # Not a git repository