mingw: follow-up to "replace isatty() hack"

mingw: follow-up to "replace isatty() hack"

The version of the "replace isatty() hack" that got merged a few
weeks ago did not actually reflect the latest iteration of the patch
series: v3 was sent out with these changes, as requested by the
reviewer Johannes Sixt:

- reworded the comment about "recycling handles"

- moved the reassignment of the `console` variable before the dup2()
  call so that it is valid at all times

- removed the "handle = INVALID_HANDLE_VALUE" assignment, as the local
  variable `handle` is not used afterwards anyway

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>