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

compat/mingw.h

@@ -395,7 +395,11 @@ HANDLE winansi_get_osfhandle(int fd);
 	(isalpha(*(path)) && (path)[1] == ':' ? 2 : 0)
 int mingw_skip_dos_drive_prefix(char **path);
 #define skip_dos_drive_prefix mingw_skip_dos_drive_prefix
-#define is_dir_sep(c) ((c) == '/' || (c) == '\\')
+static inline int mingw_is_dir_sep(int c)
+{
+	return c == '/' || c == '\\';
+}
+#define is_dir_sep mingw_is_dir_sep
 static inline char *mingw_find_last_dir_sep(const char *path)
 {
 	char *ret = NULL;