Release script fails with a newer git version

I'm using `git version 2.33.0`
```
Remote upstream URL is 'git@github.com:dprotaso/serving.git'
+ [[ -z git@github.com:dprotaso/serving.git ]]
+ setup_branch
+ [[ -z release-1.1 ]]
+ git fetch https://github.com/dprotaso/serving release-1.1:upstream/release-1.1
fatal: Refusing to fetch into current branch refs/heads/upstream/release-1.1 of non-bare repository
+ (( IS_PROW ))
+ git checkout -b release-1.1 upstream/release-1.1
warning: refname 'upstream/release-1.1' is ambiguous.
warning: refname 'upstream/release-1.1' is ambiguous.
fatal: Ambiguous object name: 'upstream/release-1.1'.
+ abort 'cannot checkout branch release-1.1'
+ echo 'error: cannot checkout branch release-1.1'
error: cannot checkout branch release-1.1
```

Start by locating the release script and its setup_branch function, then reproduce the failure with Git 2.33.0 using the fetch and checkout commands shown in the report. Done means the release script can fetch and check out release-1.1 without the non-bare repository or ambiguous ref errors.

Written by the indexing model from the issue text.