Bundle exactly the named refs (git bundle create - <ref1> <ref2> …). A
bundle of specific refs is a valid bundle: git clone of it yields those
branches and nothing else, so a non-selected branch is genuinely not in it.
Does refname resolve to an existing object in repo? Uses
git rev-parse --verify --quiet, which exits non-zero (quietly) when the
ref doesn’t exist. Any spawn failure is treated as “doesn’t exist”.