add bash completion script for repo

From Chromium [1]

[1] http://git.chromium.org/gitweb/?p=chromiumos/platform/dev-util.git;a=blob_plain;f=host/repo_bash_completion;hb=master

Change-Id: If246a455244505b26f854ba83d662341d9353592
Merge remote-tracking branch 'upstream/cm-10.1' into cm-10.1