commit: update the --secret code to use backupconfig and restoreconfig Those dedicated methods also preserve all associated data (eg: sources, lack of value).

rebase: do not retract phase boundary by hand We rely on the internal mechanism to commit the changeset in the right phase. This similar to what the mq extension is doing. This is an important change as we plan to includes phase movement within the transaction. Avoiding phase movement from high-level code will avoid the burden of transaction handling. It is also important to limit the need for transaction handling as this limits the odds of people messing up. Most common expected mess-up is code using a different transaction for changeset creation and phase adjustment.

config: fix restoreconfig of non existing config When the section, but no value existed, the `del` call raised a key error.