contrib/coccinelle: add basic Coccinelle transforms

Coccinelle (http://coccinelle.lip6.fr/) is a program which performs
mechanical transformations on C programs using semantic patches.  These
semantic patches can be used to implement automatic refactoring and
maintenance tasks.

Add a set of basic semantic patches to convert common patterns related
to the struct object_id transformation, as well as a README.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/coccinelle/README

@@ -0,0 +1,2 @@
+This directory provides examples of Coccinelle (http://coccinelle.lip6.fr/)
+semantic patches that might be useful to developers.

contrib/coccinelle/object_id.cocci

@@ -0,0 +1,95 @@
+@@
+expression E1;
+@@
+- is_null_sha1(E1.hash)
++ is_null_oid(&E1)
+
+@@
+expression E1;
+@@
+- is_null_sha1(E1->hash)
++ is_null_oid(E1)
+
+@@
+expression E1;
+@@
+- sha1_to_hex(E1.hash)
++ oid_to_hex(&E1)
+
+@@
+expression E1;
+@@
+- sha1_to_hex(E1->hash)
++ oid_to_hex(E1)
+
+@@
+expression E1;
+@@
+- sha1_to_hex_r(E1.hash)
++ oid_to_hex_r(&E1)
+
+@@
+expression E1;
+@@
+- sha1_to_hex_r(E1->hash)
++ oid_to_hex_r(E1)
+
+@@
+expression E1;
+@@
+- hashclr(E1.hash)
++ oidclr(&E1)
+
+@@
+expression E1;
+@@
+- hashclr(E1->hash)
++ oidclr(E1)
+
+@@
+expression E1, E2;
+@@
+- hashcmp(E1.hash, E2.hash)
++ oidcmp(&E1, &E2)
+
+@@
+expression E1, E2;
+@@
+- hashcmp(E1->hash, E2->hash)
++ oidcmp(E1, E2)
+
+@@
+expression E1, E2;
+@@
+- hashcmp(E1->hash, E2.hash)
++ oidcmp(E1, &E2)
+
+@@
+expression E1, E2;
+@@
+- hashcmp(E1.hash, E2->hash)
++ oidcmp(&E1, E2)
+
+@@
+expression E1, E2;
+@@
+- hashcpy(E1.hash, E2.hash)
++ oidcpy(&E1, &E2)
+
+@@
+expression E1, E2;
+@@
+- hashcpy(E1->hash, E2->hash)
++ oidcpy(E1, E2)
+
+@@
+expression E1, E2;
+@@
+- hashcpy(E1->hash, E2.hash)
++ oidcpy(E1, &E2)
+
+@@
+expression E1, E2;
+@@
+- hashcpy(E1.hash, E2->hash)
++ oidcpy(&E1, E2)