submit_branch automatic handling is confusing

[Vincent Ladeuil]

Discussed on the mailing list recently, highlights included:

- merge sets submit_branch by default to a value that it not appropriate for all workflows,

- there is no way to instruct merge to *not* set the option

- that merge is using submit_branch is unclear for many users

Proposals:

- respect --no-remember when the option is not set (i.e. --no-remember always means don't remember, whether or not the option is set)

- create a new option merge_location, merge_branch or bzr.merge.from specific to merge