mingw.h: permit arguments with side effects for is_dir_sep

mingw.h: permit arguments with side effects for is_dir_sep

Taking git-compat-util.h's cue (which uses an inline function to back
is_dir_sep()), let's use an inline function to back also the Windows
version of is_dir_sep(). This avoids problems when calling the function
with arguments that do more than just provide a single character, e.g.
incrementing a pointer. Example:

    is_dir_sep(*p++)

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>