Fetching HEAD creates a local HEAD branch

**Describe the bug**
Selecting `/HEAD` in the branch-selection dialog creates a local `HEAD` branch, which leads to:

`warning: refname 'HEAD' is ambiguous.`

**To Reproduce**
Steps to reproduce the behavior:
1. Go to 'Branches' dialog (`b`)
2. Select 'Remote' (`Tab`)
3. Scroll down to (for example) 'upstream/HEAD'
4. Press Enter
5. Exit program & run, for example, `git branch`

**Expected behavior**
Program doesn't create branches called `HEAD`.

**Context (please complete the following information):**
- OS/Distro + Version: `Ubuntu 25.04`
- GitUI Version `gitui nightly 2025-01-15 (99f6967)` (snap)