Unable to clone git repo

[Tim Haines]

I'm  not sure when this started, but our CI jobs failed last night and the
issue is still happening.

$ git clone --depth=1 https://sourceware.org/git/systemtap.git
Cloning into 'systemtap'...
warning: remote HEAD refers to nonexistent ref, unable to checkout.

Explicitly specifying the branch works.

$ git clone --depth=1 --branch=master
https://sourceware.org/git/systemtap.git
Cloning into 'systemtap'...
remote: Enumerating objects: 5288, done.
remote: Counting objects: 100% (5288/5288), done.
remote: Compressing objects: 100% (3743/3743), done.
remote: Total 5288 (delta 1524), reused 4194 (delta 1405), pack-reused 0
Receiving objects: 100% (5288/5288), 7.17 MiB | 13.76 MiB/s, done.
Resolving deltas: 100% (1524/1524), done.

Many thanks.
- Tim