Allow bootstrap to use default remote branch

[spiffxp]

I'm on the fence about whether we should do this, since we consider bootstrap to be migrated.

Many bootstrap jobs use --repo=org_name/repo_name=master when what they really mean is "use the remote's default branch"

$ ag \\-\\-repo= | grep =master | wc -l
      98

Unfortunately --repo=org_name/repo_name will default to master rather than detecting the default branch

$ ag \\-\\-repo= | grep -v master | grep -v PULL_REFS | grep -v =release | wc -l
     100

We need to either:

• solve for pod-utils (ref: Allow pod-utils / clonerefs to use default remote branch #20667) and insist jobs migrate to it
• OR drop the =master bit and update bootstrap to detect the default remote branch

/sig testing
/area jobs
/area bootstrap
/sig release
(a lot of the latter case is for k8s.io/release)
/wg naming
since this is in support of kubernetes/org#2222

[BenTheElder]

if the patch is relatively small, the utility of unblocking this migration is higher than gating it on migrating off of bootstrap

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