`ZULIP_GIT_REF` only allows fully qualified git ref name

[n0099]

https://stackoverflow.com/questions/1526471/git-difference-between-branchname-and-refs-heads-branchname

$ docker compose build zulip
[+] Building 80.8s (9/17)
 => [zulip internal] load build definition from Dockerfile                                                                                                         0.1s
 => => transferring dockerfile: 2.86kB                                                                                                                             0.0s
 => [zulip internal] load .dockerignore                                                                                                                            0.1s
 => => transferring context: 143B                                                                                                                                  0.0s
 => [zulip internal] load metadata for docker.io/library/ubuntu:20.04                                                                                              2.5s
 => [zulip base 1/2] FROM docker.io/library/ubuntu:20.04@sha256:33a5cc25d22c45900796a1aca487ad7a7cb09f09ea00b779e3b2026b4fc2faba                                   0.1s
 => => resolve docker.io/library/ubuntu:20.04@sha256:33a5cc25d22c45900796a1aca487ad7a7cb09f09ea00b779e3b2026b4fc2faba                                              0.0s
 => => sha256:33a5cc25d22c45900796a1aca487ad7a7cb09f09ea00b779e3b2026b4fc2faba 1.13kB / 1.13kB                                                                     0.0s
 => => sha256:3246518d9735254519e1b2ff35f95686e4a5011c90c85344c1f38df7bae9dd37 424B / 424B                                                                         0.0s
 => => sha256:6df89402372646d400cf092016c28066391a26f5d46c00b1153e75003465484d 2.30kB / 2.30kB                                                                     0.0s
 => [zulip internal] load build context                                                                                                                            0.0s
 => => transferring context: 23.86kB                                                                                                                               0.0s
 => [zulip base 2/2] RUN { [ ! "$UBUNTU_MIRROR" ] || sed -i "s|http://\(\w*\.\)*archive\.ubuntu\.com/ubuntu/\? |$UBUNTU_MIRROR |" /etc/apt/sources.list; } &&     30.6s
 => [zulip build 1/6] RUN useradd -d /home/zulip -m zulip &&     echo 'zulip ALL=(ALL:ALL) NOPASSWD:ALL' >> /etc/sudoers                                           0.5s
 => [zulip build 2/6] WORKDIR /home/zulip                                                                                                                          0.1s
 => ERROR [zulip build 3/6] RUN git clone "https://github.com/zulip/zulip.git" &&     cd zulip &&     git checkout -b current "zulip-cloud-current"               46.7s
------
 > [zulip build 3/6] RUN git clone "https://github.com/zulip/zulip.git" &&     cd zulip &&     git checkout -b current "zulip-cloud-current":
0.253 Cloning into 'zulip'...
45.95 fatal: 'zulip-cloud-current' is not a commit and a branch 'current' cannot be created from it
------
failed to solve: process "/bin/sh -c git clone \"$ZULIP_GIT_URL\" &&     cd zulip &&     git checkout -b current \"$ZULIP_GIT_REF\"" did not complete successfully: exit code: 128

https://git-scm.com/docs/git-checkout#Documentation/git-checkout.txt-ltstart-pointgt