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: I98570483ea251f29aca6494bb88bcae5434cd3b5
Merge branch 'jellybean' of github.com:ParanoidAndroid/android_vendor_pa into jellybean