get_oid_hex_segment(): don't pad the rest of `oid`

get_oid_hex_segment(): don't pad the rest of `oid`

Remove the feature of `get_oid_hex_segment()` that it pads the rest of
the `oid` argument with zeros. Instead, do this at the caller who
needs it.

This makes the functionality of this function more coherent and
removes the need for its `oid_len` argument.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>