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

[ch <cr@onlinehome.de>]

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