[MERGE] Create branch without working-tree

[jk]

Feature:
This adds a "--no-tree" option to the "bzr branch" command. When
present, the working-tree will not be created when the branch is
performed.

Example Usage:
bzr branch --no-tree http://bazaar-vcs.org/bzr/bzr.dev bzr.dev

Justification:
We have a few issues we're trying to overcome before we can use Bazaar
completely. This patch helps our need to create read-only local
mirrors of branches [similar to how a local mirror of bzr.dev is
created following http://bazaar-vcs.org/BzrGivingBack]. Since our
repository is 7.2Gb and the working-trees in question are 2.7Gb, it
can take 20+ minutes to create the working tree. Using this
"--no-tree" option and a shared repository, a read-only local branch
snapshot can be created in a couple of minutes or less.

Test Cases:
Never seen Python before this, and am a bit lost in the test structure
of Bazaar. So, I have no test cases. :(