NewCommitPreorderIter Fails with `object not found` When iterating over incomplete history.

[NonLogicalDev]

In remote.go in getHaves, getHavesRef fails with object not found error when
iterating over incomplete history.

This causes Fetch to fail under certain circumstances, such as (at least) when fetching remotes with unrelated histories.

This likely happens because commits are processed in Preorder and once it reaches the last commit fetched with a given Depth, it fails for find its parent.

Suggestion Short Term

Ignore object not found error during a fetch operation

Suggestion Long Term

Add a terminal empty commit after fetch with a certain depth, to signify that no further commits can be reached. This will guarantee that object not found error if it happens, happens because of corrupt data as opposed to truncated history.

[NonLogicalDev]

Seems like there is a related Issue #1143