If a user is asking for a shallow clone of the repos, they probably
expect a shallow clone of the manifest repo too. For very large
manifest repos, this can be a huge space and time savings. For one real-world
repo, a 'repo init --no-tags --current-branch' used 350MB of disk space and
took 7 minutes. Adding --depth 1 and this change reduced it to 10MB and 2.5
minutes.
Change-Id: I6fa662e174e623ede8861efc862ce26d65d4958d