Support gitfile-based repositories in gh repo create --source .

### Describe the bug

gh repo create --source . fails to detect a Git repository when the working tree uses a gitfile (.git file pointing to the actual Git directory) instead of a .git directory.

### Affected version

gh version 2.92.0 (2026-04-28)

### Steps to reproduce the behavior

1. Create a Git repository that uses a gitfile (.git is a file containing gitdir: ...).
2. Run:
gh repo create repo-name123 --private --push --source . --remote origin
3. Observe the error:
current directory is not a git repository. Run 'git init' to initialize it

### Expected vs actual behavior

Expected: gh should recognize repositories that use a gitfile, the same way Git does.

Actual: gh reports that the current directory is not a Git repository.