init: use --current-branch by default

People rarely care about having all manifest branches locally.  Change
the default to only pull down the selected branch.  If people want other
branches, the -b option will fetch it automatically, or people can use
--no-current-branch.

This only applies to the manifest project syncing, not the rest of the
projects that are in the checkout.

Bug: https://crbug.com/gerrit/16194
Change-Id: Ia9e7e2f23b8028d82772db72dcc7d6c32260be79
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/343434


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