Merge branch 'mh/reporting-broken-refs-from-for-each-ref' into maint

"git for-each-ref" reported "missing object" for 0{40} when it encounters a broken ref. The lack of object whose name is 0{40} is not the problem; the ref being broken is. * mh/reporting-broken-refs-from-for-each-ref: read_loose_refs(): treat NULL_SHA1 loose references as broken read_loose_refs(): simplify function logic for-each-ref: report broken references correctly t6301: new tests of for-each-ref error handling