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 <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-revindex.c

diff --git a/pack-revindex.c b/pack-revindex.c
index a508d5f..5e69bc7 100644
--- a/pack-revindex.c
+++ b/pack-revindex.c
@@ -177,21 +177,21 @@ int load_pack_revindex(struct packed_git *p)
 int offset_to_pack_pos(struct packed_git *p, off_t ofs, uint32_t *pos)
 {
 	unsigned lo, hi;
-	const struct revindex_entry *revindex;
 
 	if (load_pack_revindex(p) < 0)
 		return -1;
 
 	lo = 0;
 	hi = p->num_objects + 1;
-	revindex = p->revindex;
 
 	do {
 		const unsigned mi = lo + (hi - lo) / 2;
-		if (revindex[mi].offset == ofs) {
+		off_t got = pack_pos_to_offset(p, mi);
+
+		if (got == ofs) {
 			*pos = mi;
 			return 0;
-		} else if (ofs < revindex[mi].offset)
+		} else if (ofs < got)
 			hi = mi;
 		else
 			lo = mi + 1;