git-xargs doesn't update existing PRs

**Describe the bug**
First of all, I recently discovered this amazing tool and I've used it to open a bunch of PRs across a few repos. That worked amazingly!

Now I got some review feedback and I'm trying to update those PRs by using git-xargs again. Here's what I did:

```
git-xargs \
--branch-name "test-branch1" \
--commit-message "Upgrade jira-description-action to use node16" \
--repos ~/repos.txt \
--pull-request-title "SINF-522 Upgrade jira-description-action to use node16" \
--pull-request-description "Because the latest version uses node 16" \
--loglevel DEBUG \
/tmp/2.sh
```

The intention is that I want git-xargs to run `/tmp/2.sh` to addresses the feedback on the existing PR branch(es) and push the changes. That's the behavior of git-xargs when the branch already exists. However, git-xargs seems to ignore the existing remote branch and creates a new local branch with the same name. When it tries to push the branch to remote, it will report this error:

`Error="non-fast-forward update"`

Why is git-xargs not making changes to the existing branch?

**To Reproduce**
See above

```hcl
// paste code snippets here
```

**Expected behavior**
git-xargs should make changes to the existing branch.

**Nice to have**
- [ ] Terminal output
- [ ] Screenshots

**Additional context**
Add any other context about the problem here.