[MERGE] Restore default stacking policy support

[John Arbash Meinel]

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Aaron Bentley wrote:
> Hi all,
> 
> This patch restores support for stacking policies to the branch command.

To be pedantic, bzr.dev never had stacking policies removed. Only the 1.6
branch. I *am* happy to see this cleaned up.

+    * ``bzr branch`` now respects default stacking policies.  (Aaron Bentley)
+

^- ``bzr branch`` uses the default stacking policy if the branch format
   supports it.

Perhaps?

>  I've reorganized the code a bit so that:
> 
> 1. The target bzrdir has full control over the formats of branches and
>    repositories it creates.
> 2. Formats are only overridden if the user specifies --stack, not when
>    there is a default stacking policy.
> 
> Unfortunately, Branch.sprout likes to access the repository, and this
> happens before stacking can be configured on the branch.  I have
> overridden this for now, but I think we probably need to look at a
> different approach to Sprout.  Perhaps extending RepositoryPolicies so
> that they can be configured with all the requirements would make sense.
>  Other ideas are welcome.
> 
> In the process, I've discovered that --1.6-rich-root uses the wrong
> serializer.  This means that it cannot stack on top of rich-root-pack.
> It also means that it may not reliably refuse inventories containing
> subtrees.  I have filed a bug here:
> https://bugs.launchpad.net/bzr/+bug/262333
> 
> Aaron

+    def _get_metadir(self, require_stacking=False):

^- I'm not sure about the name of this function. It doesn't seem clear what
the difference is between this and "cloning_metadir". Is there a reason to not
just update the api of "cloning_metadir" to take a "require_stacking" parameter?

...
+            cloning_format.get_branch_format().supports_stacking()):
+            # We need to make a stacked branch, but the default format for the
+            # target doesn't support stacking.  So force a branch that *can*
+            # support stacking.
+            from bzrlib.branch import BzrBranchFormat7
+            cloning_format._branch_format = BzrBranchFormat7()

^- I realize this is somewhat necessary, but it does feel a bit odd to be
hard-coding Format7. I know we have the bug open about 'subtree/rich-root',
but as we add more branch formats it seems like we have to keep updating this.
Would it be possible to have a single recommended-for-stacking registry rather
than hard-coding this? (This is not a blocking issue, this is just being
raised for discussion.)

Otherwise it seems fine, I *do* like having a single function for upgrading to
a stackable BzrDir, rather than having the functionality spread into multiple
places.

BB:tweak

John
=:->
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