Merge branch 'js/gc-with-stale-symref' into maint

"git gc" used to barf when a symbolic ref has gone dangling (e.g. the branch that used to be your upstream's default when you cloned from it is now gone, and you did "fetch --prune"). * js/gc-with-stale-symref: pack-objects: do not get distracted by broken symrefs gc: demonstrate failure with stale remote HEAD