amend: support "history-editing-backup" config option
Now, amend is in the list of those history editing commands
which support history-editing-backup config option.
If you don't want to store any backup then just use this config.
hisotry-editing-backup = False
Current status of list of history editing commands which support
Differential Revision: https://phab.mercurial-scm.org/D3968