Bound branch implementation

[Nathaniel McCallum]

Call me crazy (feel free to do so), but let me brainstorm for a second.
What about this?  It seems more consistent and it lessens (by one) the
top level command namespace (which is already overcrowded).  Basically,
I'm introducing the idea of "shadow" branches which we can push changes
to or sync with (bi-directional).

* change the syntax for merge to:
 - bzr merge <from_URI> <to_URI>   (merge two arbitrary branches)
 - bzr merge -t|--to <URI>         (does: "bzr merge ./ <to>")
 - bzr merge -f|--from <URI>       (does: "bzr merge <from> ./")
 - bzr merge -u|--update [<URI>]   (does: "bzr merge <URIparent> <URI>")
 - bzr merge -s|--sync <URI> <URI> (performs a bi-directional merge)
 - bi-directional merges do "bzr merge <a> <b>" then "bzr merge <b> <a>"
 - refuse merge if any uncommitted changes in any related branch
 - handle conflicts like a normal commit.  ie:
   $ bzr merge -s ./ /foo
     > There are conflicts!  Fix them, commit them, then re-merge...
   <fix conflicts>
   $ bzr commit -m "fixed merge conflicts"
   $ bzr merge -s ./ /foo
* drop pull (use "bzr merge --update")
* drop push (use "bzr merge --to /foo")
* add the following:
 - bzr shadows -a|--add <URI>      (add URI to the shadow registry)
 - bzr shadows -d|--del <URI>      (del URI from the shadow registry)
 - bzr shadows -l|--list           (lists registered shadows/branch)
 - bzr shadows -u|--update         (foreach: "bzr merge -t <shadow>")
 - bzr shadows -s|--sync           (foreach: "bzr merge -s ./ <shadow>")
 - bzr commit  -u|--update-shadows (calls "bzr shadows -u" after commit)
 - bzr commit  -s|--sync-shadows   (calls "bzr shadows -s" after commit)

* Thus, the "bound branch" case is this:
  $ bzr branch sftp://someone.elses/branch mybranch
  $ cd mybranch
  $ bzr shadows -a sftp://someone.elses/branch  # $parent ?
  <hack hack hack>
  $ bzr commit -s -m "my changes"

* To break away from the "binding":
  <out of the office>
  <hack hack hack>
  $ bzr commit -m "my vacation changes"
  <back at work>
  $ bzr shadows -s

* Having a public mirror of your archive
  $ bzr shadows -a ftp://my.public/branch/on/my/www
  $ bzr shadows -u

* To only push out a feature when it is ready:
  <start new feature, hack hack hack>
  $ bzr commit -m "started new feature"
  <finish new feature, hack hack hack>
  $ bzr commit -u -m "finished new feature"