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

This reverts commit 076d5465.

Reason for revert: crbug.com/gerrit/16358

Change-Id: I2970eb50677cca69786f71edffe4aa5271cf139f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/349834


Reviewed-by: Sam Saccone <samccone@google.com>
Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Xin Li <delphij@google.com>