load extensions only after the ui object has been completely initialized This fixes a traceback printed when hg tries to print another traceback after an extension fails to be loaded. Add a test for that.

adding dir/bar
adding foo
dir/bar
foo
adding dir/bar_2
adding foo_2
dir/bar_2
foo_2
adding a
adding c
adding b
adding d
removing a
removing c
recording removal of a as rename to b (100% similar)