get-app ignores --branch argument

[barredterra]

> bench --version
5.7.2
> bench get-app erpnext --branch version-13
Getting erpnext
$ git clone https://github.com/frappe/erpnext.git  --depth 1 --origin upstream
Cloning into 'erpnext'...
remote: Enumerating objects: 6639, done.
remote: Counting objects: 100% (6639/6639), done.
remote: Compressing objects: 100% (5320/5320), done.
remote: Total 6639 (delta 1491), reused 3346 (delta 1012), pack-reused 0
Receiving objects: 100% (6639/6639), 18.04 MiB | 6.57 MiB/s, done.
Resolving deltas: 100% (1491/1491), done.

As you can see in line 5, git pulls the default branch (develop) regardless of the --branch argument.