Change Native Windows shell

[Steven Penny <svnpenn@gmail.com>]

The Git pager is set up by the function `setup_pager` [1]. `setup_pager` calls
`prepare_pager_args` [2]. `prepare_pager_args` sets `use_shell` [3]. Then
`setup_pager` calls `start_command` [4]. Then, since `use_shell` has been
defined, `prepare_shell_cmd` is called [5]. Finally, regardless of operating
system, `sh` is called [6]:

    #ifndef GIT_WINDOWS_NATIVE
                    argv_array_push(out, SHELL_PATH);
    #else
                    argv_array_push(out, "sh");
    #endif
                    argv_array_push(out, "-c");

The issue is, that it is possible to build a fully static native Windows
Git [7]. A Git like this can run on a stock Windows system, with no requirement
on MSYS2 or Cygwin. So it doesnt make sense for Git to be calling `sh`, when a
user may not have or need `sh` on their system. I think that on Native Windows
builds, that the Pager should be called directly, or if a shell must be used,
then make it `cmd.exe` or `powershell.exe`.

1. https://github.com/git/git/blob/20514004/pager.c#L106
2. https://github.com/git/git/blob/20514004/pager.c#L127
3. https://github.com/git/git/blob/20514004/pager.c#L101
4. https://github.com/git/git/blob/20514004/pager.c#L130
5. https://github.com/git/git/blob/20514004/run-command.c#L928
6. https://github.com/git/git/blob/20514004/run-command.c#L272-L277
7. https://github.com/nu8/gulf/blob/e9ea5c0b/chapter-2/program.sh