Added test for git.no_git_env().

Signed-off-by: Brett Randall <javabrett@gmail.com>

pre_commit/git.py

@@ -18,7 +18,7 @@ def zsplit(s):
         return []
 
 
-def no_git_env():
+def no_git_env(_env=None):
     # Too many bugs dealing with environment variables and GIT:
     # https://github.com/pre-commit/pre-commit/issues/300
     # In git 2.6.3 (maybe others), git exports GIT_WORK_TREE while running
@@ -27,8 +27,9 @@ def no_git_env():
     # while running pre-commit hooks in submodules.
     # GIT_DIR: Causes git clone to clone wrong thing
     # GIT_INDEX_FILE: Causes 'error invalid object ...' during commit
+    _env = _env if _env is not None else os.environ
     return {
-        k: v for k, v in os.environ.items()
+        k: v for k, v in _env.items()
         if not k.startswith('GIT_') or
         k in {'GIT_EXEC_PATH', 'GIT_SSH', 'GIT_SSH_COMMAND'}
     }