init: change --depth default to 1 for manifest repo

People rarely care about the history of the manifest repo.  Change
the default to 1 to speed up initial setup.  If people really want
the full history, they can pass --manifest-depth=0.

We can't reuse the existing --depth option because that applies to
all projects we clone, not just the manifest repo.

Bug: https://crbug.com/gerrit/16193
Change-Id: Ideab1712e9ffc743b9bae97903e074d7d429b134
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/343435


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: LaMont Jones <lamontjones@google.com>