'bzr update' probes the master branch >=3 times

[John A Meinel]

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The current bzr update code probes for the master branch several times
(which includes probing for the repository format, etc.)

The locations are:

 cmd_update.run()
   Here it opens the master branch to check if the last_revision is the
   same as local, and thus 'nothing to do'.
   If they differ, it then calls WT.update() passing in possible
   transports (so we won't reconnect, but be *will* re-probe for the
   master branch.)

 WT.update()
   uses a "if self.branch.get_master_branch() is not None", which
   requires probing to grab the remote branch, and then throwing it away
   right away.
   We might consider switching to self.branch.get_bound_location(),
   which doesn't have to actually open the master branch.

 Branch.update()
   Opens the master branch and finally does something useful with it
   (pull)

 affects /products/bzr
 importance medium
 status triaged

John
=:->
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