t5500: make hash independent

t5500: make hash independent

This test has hard-coded pkt-lines with object IDs.  The pkt-line
lengths necessarily differ between hash algorithms, so generate these
lines with the packetize helper so they're always the right size.  In
addition, we will require an object-format capability for SHA-256, so
pass that capability on to the upload-pack process.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>