add bash completion script for git

From git.git, contrib/completion/git-completion.bash

Change-Id: Ic7eb3cb01b1831ec20a1d99dee15171f7c919d54
diff --git a/bash_completion/repo.bash b/bash_completion/repo.bash
index 500734a..e556d33 100644
--- a/bash_completion/repo.bash
+++ b/bash_completion/repo.bash
@@ -1,6 +1,6 @@
 # Copyright (c) 2011 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
-# found in the LICENSE file.
+# found in the repo-LICENSE file.
 
 # Programmable completion for some Chromium OS build scripts.