Support bare worktrees where worktree does not have its own .git file

This was on oversight on my part: I assumed that the --work-tree arg was
always intended for use with linked worktrees which have a .git file
pointing back to the repo.

I'm honestly confused now: seems like there are three kinds of worktrees:
* the main worktree of a non-bare repo
* a linked worktree (with its own gitdir in the repo's worktrees/ dir)
* a random folder which you specify as a worktree with the --work-tree arg

I'm pretty sure the --work-tree arg is only intended to be used with this
third kind or workree

pkg/commands/git_commands/worktree_loader.go

@@ -58,7 +58,7 @@ func (self *WorktreeLoader) GetWorktrees() ([]*models.Worktree, error) {
 			isPathMissing := self.pathExists(path)
 
 			var gitDir string
-			gitDir, err := worktreeGitDirPath(self.Fs, path)
+			gitDir, err := getWorktreeGitDirPath(self.Fs, path)
 			if err != nil {
 				self.Log.Warnf("Could not find git dir for worktree %s: %v", path, err)
 			}