[qbzr/bzr-gtk] Save commit message and other metadata when commit GUI dialog is cancelled or in uncommit hook

[Gary van der Merwe]

So something else that I think we should be thinking about here:

I've been using bzr-builddeb a bit lately. One of the things that it
does, is pull what you have added to debian/changelog and adds it to
you commit file (that opens in you editor if you bzr commit with out
-m.) But if you do qcommit or gcommit, it does not pull this up.

So I think we need a api to say give me the default/restored commit
message + and other metadata (--fixes, --author). The bzr cli should
read the message and write it to the message file, and ignore the
other metadata. qcommit and gcommit can use all the data.

Plugins, such as bzr-builddeb, should be able to hook in to provide
this data if appropriate.  A plugin that I would like to write is one
that will provide a default message "Merge %s." % merged_branch_nick
if you have done a merge.