import: convert init.defaultBranch to fully qualified ref
init.defaultBranch expects a branch name, not a fully qualified ref.
git-init prepends "refs/heads/" automatically and unconditionally.
PublicInbox::Import::default_branch, however, incorrectly passes on
the init.defaultBranch value as is, leading to it being used in spots
where a fully qualified ref is required. For example, with an
init.defaultBranch value of "master", public-inbox-index for a v2
repository would lead to an all.git repository where HEAD's content is
"ref: master" instead of "ref: refs/heads/master".
Prepend "refs/heads/" to the incoming init.defaultBranch value.