Bug: branch name shows `.invalid` with git reftable

[none23]

On git repos with refstorage set to reftable, branch name displays as .invalid.

It is because .git/HEAD has ref: refs/heads/.invalid and cannot be used as a source for branch names

Self Checks

• I'm using the latest lualine.
• I didn't find the issue in existing issues or PRs.

How to reproduce the problem

1. Create a git repo with reftable enabled:

[core]
	repositoryformatversion = 1
[extensions]
	refstorage = reftable

2. Open neovim

Expected behaviour

Actual branch name is displayed

Actual behaviour

Branch name is .invalid

Minimal config to reproduce the issue

Additional information