extensions: clear aftercallbacks after execution (issue4646) It was reported that enabling pager without color could cause a hang. Inserting print statements revealed that the callbacks in extensions._aftercallbacks were being invoked twice. extensions.loadall can be called multiple times. If entries in extensions._aftercallbacks linger between calls, this could result in double execution of the callbacks. This can lead to unwanted behavior. The reproduce steps in the bug seem to only occur when the output of a command is less than the size of the current screen. This is not something that can easily be tested. I verified the test case works with this patch and that pager and color interaction continues to work. Since we have no existing automated tests for pager, this sadly appears to be the best testing I can do.