mingw: avoid the comma operator

mingw: avoid the comma operator

The pattern `return errno = ..., -1;` is observed several times in
`compat/mingw.c`. It has served us well over the years, but now clang
starts complaining:

  compat/mingw.c:723:24: error: possible misuse of comma operator here [-Werror,-Wcomma]
    723 |                 return errno = ENOSYS, -1;
        |                                      ^

See for example this failing workflow run:
https://github.com/git-for-windows/git-sdk-arm64/actions/runs/15457893907/job/43513458823#step:8:201

Let's appease clang (and also reduce the use of the no longer common
comma operator).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>