coccinelle: introduce drop-braces transformation

to drop braces around single-line if statements. Also, prefix it with
zz- so it runs as the last one, so it's able to fix stuff tweaked by
previous transformations.

coccinelle/zz-drop-braces.cocci

@@ -0,0 +1,27 @@
+@@
+position p : script:python() { p[0].file != "src/journal/lookup3.c" };
+identifier id;
+expression e;
+@@
+if (...)
+- {
+(
+    id@p(...);
+|
+    e@p;
+)
+- }
+
+@@
+position p : script:python() { p[0].file != "src/journal/lookup3.c" };
+identifier id;
+expression e;
+@@
+if (...)
+- {
+(
+    return id@p(...);
+|
+    return e@p;
+)
+- }