Wrong git branch is displayed when using a linked git worktree

[bacar]

When using git worktrees, the git branch displayed in a prompt when in a linked worktree is incorrect - it displays the current branch from the main worktree.

[bacar]

In any fix, is it worth using the plumbing commands (instead of parsing the gitdir contents directly) to make sure this is also more future-proof?

[bacar]

The problem seems to be with this line in local function has_git_file(dir):

return git_dir and dir..'/'..git_dir

This probably works for submodules (which seem to put a relative gitdir: into the .git file), but not for worktrees (which seem to put in an absolute path).