Merge branch 'jk/write-broken-index-with-nul-sha1'

Earlier we started rejecting an attempt to add 0{40} object name to the index and to tree objects, but it sometimes is necessary to allow so to be able to use tools like filter-branch to correct such broken tree objects. * jk/write-broken-index-with-nul-sha1: write_index: optionally allow broken null sha1s