bug#65763: Error opening a file from a Git working directory if Git is not installed

[Paul Pogonyshev]

To reproduce:

1. Change current directory to a Git checkout (e.g. `~/git/emacs' or something).
2. From the command line:

       $ emacs --batch --eval "(progn (setf vc-git-program \"git-is-not-installed\") (find-file-noselect \"whatever\"))"
       Error: (file-missing "Searching for program" "No such file or directory" "git-is-not-installed")

Expected: Emacs is able to open the file, Git must not be essential.

Usecase: on the real machine Git _is_ of course installed, but then you run Emacs inside a Docker container that has _no_ Git, with a directory mapped from your physical machine.  In this setup there is appearance of a Git working directory, yet Git executable is not available.

Appears to work fine up to Emacs 27, broken starting with Emacs 28.

Paul