Bug: Parser crashes on thin packs

[jpeletier]

When the parser is fed a so-called "thin pack", crashes in the get() function since it will receive a nil in the Parent objInfo property. A "thin pack" is a pack that references a SHA1 that is not contained in the pack and that the other side knows you must have.
The portion of the code that handles thin packs is not covered.

I have a patch for this that works and I will submit shortly, but I'd need help in writing tests (I need a fixture that produces thin packs)