How do I "undo" a --single-branch clone?

For me worked:

git remote remove origin
git remote add origin https://*<yourPath>*.git
git fetch

If you want to add a single branch, you can do the following:

git remote set-branches --add origin [remote-branch]
git fetch origin [remote-branch]:[local-branch]

Works with git version 1.9.1

You can tell Git to pull all branches like this:

git config remote.origin.fetch "+refs/heads/*:refs/remotes/origin/*"
git fetch origin

If you look in .git/config, it'll look something like this:

[core]
    repositoryformatversion = 0
    filemode = true
    bare = false
    logallrefupdates = true
    ignorecase = true
    precomposeunicode = false
[remote "origin"]
    url = https://github.com/owner/repo.git
    fetch = +refs/heads/*:refs/remotes/origin/*
[branch "master"]
    remote = origin
    merge = refs/heads/master
    rebase = true

I compared this to a full clone, and saw that the only difference was the "fetch" under [remote "origin"].

Note: I'm running Git version 1.8.2. The config options may have changed if you're running an older version of Git. If my commands don't work, then I'd recommend looking through .git/config to see if you can see something similar.

To add another remote branch to my local repository that was cloned using --single-branch, the following works for me:

git remote set-branches --add origin [remote-branch]
git fetch
git checkout [remote-branch]

You can also use wildcards for [remote-branch], e.g.

git remote set-branches --add origin release-1.*
git fetch
git checkout release-1.5

This works using git version 2.21.1. Other answers suggesting to do git fetch origin [remote-branch]:[local-branch] did not work as it creates the local branch in an untracked state. When running git pull it first tried to merge all the commits of the remote branch to my local one once again.