Make airline aware of git worktrees

Currently vim-airline assumes, that the git_dir is part of the path for
the file being edited. This has changed, since git supports worktrees.

So take care of b:git_dir (which is set by fugitive) being a path
differently from the absolute path of the file being edited (however, it
should include the substring worktree in that case).

autoload/airline/extensions/branch.vim

@@ -351,7 +351,13 @@ function! s:check_in_path()
         let root = expand(fnamemodify(root, ':h'))
       else
         " else it's the newer format, and we need to guesstimate
-        let pattern = '\.git\(\\\|\/\)modules\(\\\|\/\)'
+        " 1) check for worktrees
+        if match(root, 'worktrees') > -1
+          " worktree can be anywhere, so simply assume true here
+          return 1
+        endif
+        " 2) check for submodules
+        let pattern = '\.git[\\/]\(modules\)[\\/]'
         if match(root, pattern) >= 0
           let root = substitute(root, pattern, '', '')
         endif