reposync: only print updated repos during sync

repo sync redirects updated repositories information to the error
stream, so we can send stdout to /dev/null and the terminal will only
display updated repositories.

Change-Id: I00b15ac18519c9c1efbdeecdae2bcc9b167cf3f4

Conflicts:
	envsetup.sh
Merge remote-tracking branch 'ls/kitkat' into kitkat