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