Win32: unify environment case-sensitivity

The environment on Windows is case-insensitive. Some environment functions
(such as unsetenv and make_augmented_environ) have always used case-
sensitive comparisons instead, while others (getenv, putenv, sorting in
spawn*) were case-insensitive.

Prevent potential inconsistencies by using case-insensitive comparison in
lookup_env (used by putenv, unsetenv and make_augmented_environ).

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Stepan Kasal <kasal@ucw.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -1199,8 +1199,7 @@ static int lookup_env(char **env, const char *name, size_t nmln)
 	int i;
 
 	for (i = 0; env[i]; i++) {
-		if (0 == strncmp(env[i], name, nmln)
-		    && '=' == env[i][nmln])
+		if (!strncasecmp(env[i], name, nmln) && '=' == env[i][nmln])
 			/* matches */
 			return i;
 	}