Add Coccinelle usage: one for blacklisting, one for patch collection

This is inspired by the [Coccinelle](http://coccinelle.lip6.fr/) usage
in systemd.  I also took it a bit further and added infrastructure
to have spatches which should never apply.  This acts as a blacklist.

The reason to do the latter is that coccinelle is *way* more powerful than the
regular expresssions we have in `make syntax-check`.

I started with blacklisting `g_error_free()` directly. The reason that's bad is
it leaves a dangling pointer.

Closes: #754
Approved by: jlebon

tests/coccinelle.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+
+# Run the .cocci files in the tests directory; these act
+# as a blacklist.
+
+set -euo pipefail
+
+. $(dirname $0)/libtest.sh
+
+if ! spatch --version 2>/dev/null; then
+    skip "no spatch; get it from http://coccinelle.lip6.fr/"
+fi
+
+if test -z "${OSTREE_UNINSTALLED_SRCDIR:-}"; then
+    skip "running installed?"
+fi
+
+coccitests=$(ls $(dirname $0)/coccinelle/*.cocci)
+echo "1.."$(echo ${coccitests} | wc -l)
+
+for cocci in $(dirname $0)/coccinelle/*.cocci; do
+    echo "Running: ${cocci}"
+    spatch --very-quiet --dir ${OSTREE_UNINSTALLED_SRCDIR} ${cocci} > cocci.out
+    if test -s cocci.out; then
+        sed -e 's/^/# /' < cocci.out >&2
+        fatal "Failed semantic patch: ${cocci}"
+    fi
+    echo ok ${cocci}
+done

tests/coccinelle/README.md

@@ -0,0 +1,2 @@
+Add patches here which should never match in the code; i.e. the suggested
+replacement may be junk.

tests/coccinelle/raw-free.cocci

@@ -0,0 +1,5 @@
+@@
+expression p;
+@@
+- g_error_free (p);
++ g_clear_error (&p);

tests/test-rollsum-cli.c

@@ -22,10 +22,12 @@
 
 #include "ostree-rollsum.h"
 
+#include "libglnx.h"
+
 int
 main (int argc, char **argv)
 {
-  GError *local_error = NULL;
+  g_autoptr(GError) local_error = NULL;
   GError **error = &local_error;
   GBytes *from_bytes = NULL;
   GBytes *to_bytes = NULL;
@@ -64,7 +66,6 @@ main (int argc, char **argv)
   if (local_error)
     {
       g_printerr ("%s\n", local_error->message);
-      g_error_free (local_error);
       return 1;
     }
   return 0;