use $GIT_DIR when set

Author: DoomHammer

git/repo/base.py

@@ -141,7 +141,7 @@ def __init__(self, path=None, odbt=DefaultDBType, search_parent_directories=Fals
         self.working_dir = None
         self._working_tree_dir = None
         self.git_dir = None
-        curpath = epath
+        curpath = os.getenv('GIT_DIR', epath)
 
         # walk up the path to find the .git dir
         while curpath: