Author: Simey Lameze
MDL-65044 core: add task for grade history cleanup
http://git.moodle.org/gw?p=moodle.git;a=commit;h=5acef15175ffd3d4da17cadd860a7a2f21511ba3
http://github.com/moodle/moodle/commit/5acef15175ffd3d4da17cadd860a7a2f21511ba3
+ 3.7 en [taskgradehistorycleanup,core_admin]