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.

mercurial/extensions.py

@@ -134,6 +134,10 b' def loadall(ui):'
         for fn in _aftercallbacks[shortname]:
             fn(loaded=False)
 
+    # loadall() is called multiple times and lingering _aftercallbacks
+    # entries could result in double execution. See issue4646.
+    _aftercallbacks.clear()
+
 def afterloaded(extension, callback):
     '''Run the specified function after a named extension is loaded.