compat/setenv.c: error if name contains '='

According to POSIX, setenv should error out with EINVAL if it's
asked to set an environment variable whose name contains an equals
sign. Implement this detail in our compatibility-fallback.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/setenv.c

@@ -6,7 +6,7 @@ int gitsetenv(const char *name, const char *value, int replace)
 	size_t namelen, valuelen;
 	char *envstr;
 
-	if (!name || !value) {
+	if (!name || strchr(name, '=') || !value) {
 		errno = EINVAL;
 		return -1;
 	}