given a packfile, create a stream to parse it's index file.
currently only supports v2 idx files.
var packidx =fs
return a writable stream that emits a single
'data' event containing a
attempt to find
objectID in this index file. returns either an
offset object, or
null if the
objectID isn't present in this pack index.
if an offset object is returned, it'll look like so:
offset: number | hi: number lo: numbernext: number | hi: number lo: number | null