plumbing/format/packfile: Fix broken "thin" packfile support. Fixes #991

[jpeletier]

We have noticed that thin-pack support by the parser stopped working after upgrading from 4.5.0 to the latest version 4.7.1, but going back, it doesn't work since 4.6.0.
When the parser is given a so-called "thin" pack file, it will panic with a nil-pointer exception in Parser.get() as this function receives a nil o *objectInfo parameter. I traced this back to indexObjects() case plumbing.REFDeltaObject where the Parent property is left uninitialized when a REF delta object base is not found in the current pack. Unit tests did not check for thin-pack eventuality, and thus this case was uncovered.

This is an attempt to fix the issue. The strategy is to initialize objInfo.Parent to a "placeholder" parent, so we can detect this during the resolve phase.

I would need your guidance in writing tests for this one, as I am not sure how to produce a thin pack file out of the fixtures: these types of pack files only exsist on the network when git is moving stuff, never stored on disk.

[jfontan]

I've managed to create a thin pack from fixtures repo labeled worktree, dirty:

https://github.com/src-d/go-git-fixtures/blob/v3.1.1/fixtures.go#L144

It's 726 bytes so it may be added to the test source. You can find the file here: https://downloads.zooloo.org/thin.pack

The way it was generated was creating a new commit, getting the list of unpacked objects and passing it to git pack-objects:

find .git/objects -type f | grep -v -E '(pack|info)' | sed 's|.git/objects/||' | tr -d '/' | git pack-objects --thin --stdout > thin-pack

[jpeletier]

@jfontan I have added a test for this, please take a look.
I was not able to use your method above to create a thin packfile. Upon exploring the generated packfile, I detected it actually was embedding the dependencies in.
To create a truly thin packfile I resorted to using git bundle to produce a differential bundle, then removing the bundle header. The result is the new thinpack fixture.
I had to add that new fixture to the fixtures repo, so this won't work until src-d/go-git-fixtures#11 is merged

Please check it out and let me know. Thanks.