Don't clobber the PATH environment variable when running git commit.

testing_support/git/repo.py

Index: testing_support/git/repo.py
diff --git a/testing_support/git/repo.py b/testing_support/git/repo.py
index 29cc70fc609341895933c4e3ea0bd261541d91bf..5944f16d2e660e806bc1dd728aed8295f3bf73ae 100644
--- a/testing_support/git/repo.py
+++ b/testing_support/git/repo.py
@@ -106,7 +106,8 @@ class GitRepo(object):
       self.git('checkout', '--orphan', 'root_%s' % commit.name)
       self.git('rm', '-rf', '.')
 
-    env = self.get_git_commit_env(commit_data)
+    env = os.environ.copy()
+    env.update(self.get_git_commit_env(commit_data))
 
     for fname, file_data in commit_data.iteritems():
       deleted = False
@@ -167,7 +168,9 @@ class GitRepo(object):
       return self.COMMAND_OUTPUT(e.returncode, e.output)
 
   def git_commit(self, message):
-    return self.git('commit', '-am', message, env=self.get_git_commit_env())
+    env = os.environ.copy()
+    env.update(self.get_git_commit_env())
+    return self.git('commit', '-am', message, env=env)
 
   def nuke(self):
     """Obliterates the git repo on disk.