[REGRESSION] gitk can't be run from non-worktree folders

[ch <cr@xxxxxxxxxxxxx>]

Hi all!

I recently updated my Git installation from 2.24.1.windows.2 to
2.25.0.windows.1 and from the looks of it the included version of gitk does not
support being run from non-worktree folders of a repository anymore (like the
.git folder for example).

Here's a small reproduction recipe:

  $ git init repro
  $ cd repro
  $ git commit -m test --allow-empty
  $ cd .git
  $ gitk --all &

gitk terminates with the message:

  fatal: this operation must be run in a work tree
      while executing
  "exec git rev-parse --show-toplevel"
      invoked from within
  "set worktree [exec git rev-parse --show-toplevel]"
      (file "C:/Program Files/Git/mingw64/bin/gitk" line 12634)

-ch