Tree walk: bugfix

[asayers]

From https://github.com/alexcrichton/git2-rs/pull/343:

I'm seeing some strange behaviour with this feature. I have a callback which returns TreeWalkResult::Ok unconditionally, and I expect every node in the tree to be visited. Instead I see a subset of the top-level nodes only, and this subset is not deterministic across runs! Looks like something nasty is happening. Returning 0 from the callback instead gives the expected behaviour.

I haven't been able to observe this behaviour in this branch.

[alexcrichton]

Thanks!

[asayers]

Tree::walk() should now be safe, but treewalk_cb is unsafe. However, I can't mark it as unsafe, because raw::git_tree_walk expects a safe function. Options:

• Document the invariant and cross fingers.
• Monomorphise tree::Walk and force the user to return TreeWalkResult.
• Some way to mark treewalk_cb as unsafe that I'm not thinking of?

[alexcrichton]

Ah that's ok, it's an internal function so it should be fine to leave as "safe", but adding documentation would also be great!