MINOR: hlua_fcn: add Patref:prepare() method

Just like the "prepare map" or "prepare acl" on the cli, but for Lua:
it leverages the pattern API to create a subset (ie: a new generation id)
that will automatically be used as target for following Patref operations
(add/set/del...) until the "commit" method is invoked to atomically push
the pending updates.

doc/lua-api/index.rst

@@ -3456,6 +3456,12 @@ Patref class
   :returns: true if the pattern reference is used to handle maps instead
    of acl, false otherwise.
 
+.. js:function:: Patref.prepare(ref)
+
+  Create a new empty version for Patref Object. It can be used to manipulate
+  the Patref object with update methods without applying the updates until the
+  commit() method is called.
+
 .. js:function:: Patref.commit(ref)
 
   Tries to commit pending Patref object updates, that is updates made to the
@@ -3467,6 +3473,8 @@ Patref class
 
   :returns: true on success and nil on failure (followed by an error message).
 
+  See :js:func:`Patref.prepare()`
+
 .. _applethttp_class:
 
 AppletHTTP class

src/hlua_fcn.c

@@ -2694,6 +2694,17 @@ int hlua_patref_commit(lua_State *L)
 	return _hlua_patref_clear(L, LUA_OK, 0);
 }
 
+int hlua_patref_prepare(lua_State *L)
+{
+	struct hlua_patref *ref;
+
+	ref = hlua_checkudata(L, 1, class_patref_ref);
+	BUG_ON(!ref);
+	ref->curr_gen = pat_ref_newgen(ref->ptr);
+	ref->flags |= HLUA_PATREF_FL_GEN;
+	return 0;
+}
+
 void hlua_fcn_new_patref(lua_State *L, struct pat_ref *ref)
 {
 	struct hlua_patref *_ref;
@@ -2721,6 +2732,7 @@ void hlua_fcn_new_patref(lua_State *L, struct pat_ref *ref)
 	/* set public methods */
 	hlua_class_function(L, "get_name", hlua_patref_get_name);
 	hlua_class_function(L, "is_map", hlua_patref_is_map);
+	hlua_class_function(L, "prepare", hlua_patref_prepare);
 	hlua_class_function(L, "commit", hlua_patref_commit);
 }