contrib/coccinnelle: add equals-null.cocci

Add a coccinelle semantic patch necessary to reinforce the git coding style
guideline:

"Do not explicitly compute an integral value with constant 0 or '\ 0', or a
pointer value with constant NULL."

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/coccinelle/equals-null.cocci

@@ -0,0 +1,30 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+@@
+expression e;
+statement s;
+@@
+if (
+(
+!e
+|
+- e == NULL
++ !e
+)
+   )
+   {...}
+else s
+
+@@
+expression e;
+statement s;
+@@
+if (
+(
+e
+|
+- e != NULL
++ e
+)
+   )
+   {...}
+else s