Merge branch 'ma/split-symref-update-fix'

A leakfix. * ma/split-symref-update-fix: refs/files-backend: add `refname`, not "HEAD", to list refs/files-backend: correct return value in lock_ref_for_update refs/files-backend: fix memory leak in lock_ref_for_update refs/files-backend: add longer-scoped copy of string to list