Merge branch 'er/fast-import-dump-refs-on-checkpoint' into maint

The checkpoint command "git fast-import" did not flush updates to refs and marks unless at least one object was created since the last checkpoint, which has been corrected, as these things can happen without any new object getting created. * er/fast-import-dump-refs-on-checkpoint: fast-import: checkpoint: dump branches/tags/marks even if object_count==0