Ignore worktree with broken gitdir

References https://github.com/tpope/vim-fugitive/issues/751

plugin/fugitive.vim

@@ -271,6 +271,9 @@ function! s:configured_tree(git_dir) abort
       endif
     elseif filereadable(a:git_dir . '/gitdir')
       let worktree = fnamemodify(readfile(a:git_dir . '/gitdir')[0], ':h')
+      if worktree ==# '.'
+        unlet! worktree
+      endif
     endif
     if exists('worktree')
       let s:worktree_for_dir[a:git_dir] = worktree