write_reuse_object(): convert to new revindex API

write_reuse_object(): convert to new revindex API

First replace 'find_pack_revindex()' with its replacement
'offset_to_pack_pos()'. This prevents any bogus OFS_DELTA that may make
its way through until 'write_reuse_object()' from causing a bad memory
read (if 'revidx' is 'NULL')

Next, replace a direct access of '->nr' with the wrapper function
'pack_pos_to_index()'.

Signed-off-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>