Copy environment for subprocess execution

Git utilizes multiple environment variables to control various
behaviours. Make sure to set LC_MESSAGES on a copy of the environment
instead of discarding any variables that may be set by the user or
default shell environment such as EDITOR.

Add test to assert that when overriding GIT_EDITOR via os.environ that
the modified value will be picked up by and git commands called.

Author: Darragh Bailey

Diff for: git/cmd.py

@@ -341,8 +341,10 @@ def execute(self, command,
           cwd = self._working_dir
 
         # Start the process
+        env = os.environ.copy()
+        env["LC_MESSAGES"] = "C"
         proc = Popen(command,
-                        env={"LC_MESSAGES": "C"},
+                        env=env,
                         cwd=cwd,
                         stdin=istream,
                         stderr=PIPE,

Diff for: git/test/test_git.py

@@ -5,6 +5,7 @@
 # the BSD License: http://www.opensource.org/licenses/bsd-license.php
 
 import os
+import mock
 from git.test.lib import (TestBase,
                             patch,
                             raises,
@@ -128,3 +129,8 @@ def test_single_char_git_options_are_passed_to_git(self):
 
     def test_change_to_transform_kwargs_does_not_break_command_options(self):
         self.git.log(n=1)
+
+    def test_env_vars_passed_to_git(self):
+        editor = 'non_existant_editor'
+        with mock.patch.dict('os.environ', {'GIT_EDITOR': editor}):
+            assert self.git.var("GIT_EDITOR") == editor