[MERGE] Support ``child_submit_format`` option set in the submit branch in "bzr send".

[Ian Clatworthy]

Jelmer Vernooij wrote:

> This adds a ``child_submit_format`` option that can be set in the submit
> branch to specify what send format to use when submitting changes
> against that submit branch. This option is checked if there was no
> format specified explicitly on the command-line.

Nice.

bb:approve

Ian C.