Do not break convention when updating sys.path

Author: Xavier Verges

git/__init__.py

@@ -19,7 +19,7 @@
 def _init_externals():
     """Initialize external projects by putting them into the path"""
     if __version__ == 'git' and 'PYOXIDIZER' not in os.environ:
-        sys.path.insert(0, osp.join(osp.dirname(__file__), 'ext', 'gitdb'))
+        sys.path.insert(1, osp.join(osp.dirname(__file__), 'ext', 'gitdb'))
 
     try:
         import gitdb

test/test_installation.py

@@ -1,6 +1,7 @@
 # This module is part of GitPython and is released under
 # the BSD License: http://www.opensource.org/licenses/bsd-license.php
 
+import ast
 import os
 import subprocess
 from test.lib import TestBase
@@ -27,3 +28,8 @@ def test_installation(self, rw_dir):
         self.assertEqual(0, result.returncode, msg=result.stderr or result.stdout or "Can't build - setup.py failed")
         result = subprocess.run([self.python, '-c', 'import git'], stdout=subprocess.PIPE, cwd=self.sources)
         self.assertEqual(0, result.returncode, msg=result.stderr or result.stdout or "Selftest failed")
+        result = subprocess.run([self.python, '-c', 'import sys;import git; print(sys.path)'], stdout=subprocess.PIPE, cwd=self.sources)
+        syspath = result.stdout.decode('utf-8').splitlines()[0] 
+        syspath = ast.literal_eval(syspath)
+        self.assertEqual('', syspath[0], msg='Failed to follow the conventions for https://docs.python.org/3/library/sys.html#sys.path')
+        self.assertTrue(syspath[1].endswith('gitdb'), msg='Failed to add gitdb to sys.path')