Use MB_CUR_MAX == 6 by default

Internally, Cygwin already uses __utf8_mbtowc(), even if it still claims
to use the "ASCII" charset.

But the `MB_CUR_MAX` value (which is not actually a constant, but
dependent on the current locale) was still 1, which broke the initial
`globify()` call while parsing the the command-line in `build_argv()`
for non-ASCII arguments.

This fixes git-for-windows/git#2189 and is
needed to let Git's t7400.108(submodule with UTF-8 name) pass, which
otherwise fails with:

  ++ git submodule add './å äö'
  fatal: repo URL: '"./å äö"' must be absolute or begin with ./|../

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

newlib/libc/locale/lctype.c

@@ -25,7 +25,12 @@
 
 #define LCCTYPE_SIZE (sizeof(struct lc_ctype_T) / sizeof(char *))
 
+#ifdef __CYGWIN__
+/* Cygwin uses __utf8_mbtowc() by default, therefore mb_cur_max := 6 */
+static char	numone[] = { '\x06', '\0'};
+#else
 static char	numone[] = { '\1', '\0'};
+#endif
 
 const struct lc_ctype_T _C_ctype_locale = {
 	"ASCII",			/* codeset */