Use PrRefs.GetPRHeadLabel() instead of headBranchLabel.
Also remove headBranchLabel from CreateContext struct.
To do this, we needed a new identifier for when the head repo should be
created via a new fork of the base repo. Previously, this was done by
checking if the head repo was nil, but if we want to call
GetPRHeadLabel(), it requires a non-nil head repo to construct the
headBranchLabel. So, instead of the head repo being nil to signal
a fork, we pass a new forkHeadRepo bool in the CreateContext struct.
This also makes the decision to fork more intentional; now the decision
is made clearly instead of if the headRepo happens to be nil.
c0c5d9123d