[MERGE] Create branch without working-tree

[Aaron Bentley]

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jk wrote:
> 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.

> 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.

But if you use create the repository by doing "init-repo --no-trees",
you don't need the --no-tree option.  (or you can change an existing
repository with "touch .bzr/repository/no-working-trees")

The feature itself is okay, but you'll need tests:
1. a test in bzrlib/bzrdir_implementations to ensure that all
implementations of sprout accept and respect the no_tree parameter

2. a test in bzrlib/blackbox/test_branch.py to ensure that "bzr branch"
accepts and respects a --no-tree option

bb:resubmit

Aaron
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