Why does git submodule update fail with "fatal: remote error: upload-pack: not our ref"?

With Git and submodules, you start with a minimum of two Git repositories. One is "your" repository—the main one, which Git will call the superproject. The second Git repository is some other Git repository: there is nothing special about it at all. It's just that your superproject has in it these two parts:

• Instructions for cloning the submodule. That lets your Git run git clone if needed, during git submodule update --init for instance.

• The raw hash ID of some commit that should be in that other Git repository. Your main repository will, after cloning or running git fetch if appropriate in your clone of the other Git repository, run git checkout hash using this raw hash ID.

Your superproject is asking for the hash ID 151d94a8754b0a612315fc191c5373cc0055c13d in the Git repository you can clone from https://github.com/scopatz/nanorc.git. That commit simply does not exist in that repository, so it's not in any clone you make either.

Do you know why your superproject lists this commit hash ID, even though it does not exist? (I certainly don't.) You cannot get it from their Git, because they don't have it. That's what all these error messages are telling you.

You can try searching other repositories (or Google) for 151d94a8754b0a612315fc191c5373cc0055c13d (I just tried with Google and they can't find it). Or, if you don't really care specifically for that commit, try telling your own Git—your superproject—that it should get some other commit, one that does exist and therefore you can get.

Which commit should you get? I have no idea: that's up to you. Note that the place where your superproject lists the raw commit hash from the submodule is: in each commit. You can git checkout some commit, probably the tip of some branch, in your superproject. Then you can enter the submodule, pick a commit you like, use git checkout in that submodule—it's another Git clone, after all, so you can do any Git command there—to check out the desired commit. Then, exit the submodule (change directories back to your superproject) and run git add on the submodule path and git commit to record the new desired hash ID. This new commit now asks for that particular hash ID.