rebase: support "history-editing-backup" config option
If you don't want to store any backup while rebasing, you can
use `history-editing-backup` config option.
[ui]
history-editing-backup = # True or False
Current status of list of commands which supports this config:
1. histedit
2. rebase
Differential Revision: https://phab.mercurial-scm.org/D3887
extensions: add detailed loading information
This lets you track down what exactly is happening during extension loading,
and how long various steps took.