format.PackIndexV2.OffsetForPos does not support offsets higher than 2 GiB

[kourge]

Per the v2 pack index format description, for an entry, a 32-bit offset with its MSB set to 1 means the the 31-bit LSBs form an index to the higher offsets table, which contains zero or more unsigned 64-bit integers that represent the true offset of an entry when the offset is greater than 2 GiB.

format.PackIndexV2.OffsetForPos currently assumes that this is never the case and always casts a 32-bit offset to an int64, even if its MSB is set to 1.

[kourge]

format.PackIndexV2.OffsetForPos has been removed from the API, but the PackIndexEntry value returned by format.PackIndexV2.EntryForSha1 still has a flawed implementation for its Offset method.