Merge branch 'jk/name-pack-after-byte-representation'

Two packfiles that contain the same set of objects have traditionally been named identically, but that made repacking a repository that is already fully packed without any cruft with a different packing parameter cumbersome. Update the convention to name the packfile after the bytestream representation of the data, not after the set of objects in it. * jk/name-pack-after-byte-representation: pack-objects doc: treat output filename as opaque pack-objects: name pack files after trailer hash sha1write: make buffer const-correct