"Han-Wen Nienhuys via GitGitGadget" [off-list ref] writes:
## Test upstream-gone case. Fake this by pointing
## origin/initial-branch at a non-existing commit.
- OLD=$(git rev-parse origin/initial-branch) &&
- NEW=$ZERO_OID &&
- mv .git/packed-refs .git/old-packed-refs &&
- sed "s/$OLD/$NEW/g" <.git/old-packed-refs >.git/packed-refs &&
+ git update-ref -d refs/remotes/origin/initial-branch &&
OK, so we now "fake" upstream-gone by making the upstream actually
gone ;-) OK.
HUF=$(git rev-parse HEAD) &&