index-pack --verify: read anomalous offsets from v2 idx file

A pack v2 .idx file usually records offset using 64-bit representation
only when the offset does not fit within 31-bit, but you can handcraft
your .idx file to record smaller offset using 64-bit, storing all zero
in the upper 4-byte.  By inspecting the original idx file when running
index-pack --verify, encode such low offsets that do not need to be in
64-bit but are encoded using 64-bit just like the original idx file so
that we can still validate the pack/idx pair by comparing the idx file
recomputed with the original.

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

pack.h

@@ -41,6 +41,14 @@ struct pack_idx_option {
 
 	uint32_t version;
 	uint32_t off32_limit;
+
+	/*
+	 * List of offsets that would fit within off32_limit but
+	 * need to be written out as 64-bit entity for byte-for-byte
+	 * verification.
+	 */
+	int anomaly_alloc, anomaly_nr;
+	uint32_t *anomaly;
 };
 
 extern void reset_pack_idx_option(struct pack_idx_option *);