Fetching HEAD creates a local HEAD branch #2681

Describe the bug
Selecting <remote>/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)