pack-revindex.c: avoid direct revindex access in 'offset_to_pack_pos()'

pack-revindex.c: avoid direct revindex access in 'offset_to_pack_pos()'

To prepare for on-disk reverse indexes, remove a spot in
'offset_to_pack_pos()' that looks at the 'revindex' array in 'struct
packed_git'.

Even though this use of the revindex pointer is within pack-revindex.c,
this clean up is still worth doing. Since the 'revindex' pointer will be
NULL when reading from an on-disk reverse index (instead the
'revindex_data' pointer will be mmaped to the 'pack-*.rev' file), this
call-site would have to include a conditional to lookup the offset for
position 'mi' each iteration through the search.

So instead of open-coding 'pack_pos_to_offset()', call it directly from
within 'offset_to_pack_pos()'.

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