bzr branch should be resumable if interrupted

If you do "bzr branch http://upstream local" and cancel it half-way through, it should be possible to "cd local; bzr pull" to finish pulling.

At the moment, 'bzr branch' doesn't create a target branch until after the repository is copied. The current workaround is:

bzr branch http://upstream local
^C
cd local
bzr init
bzr pull http://upstream

This has some small issues if the source is not the default format, etc. But it does let you re-use whatever data has been downloaded.

Alternatively, if you have a local shared repository, the data is stored there, so doing:

bzr init-repo --trees repo
cd repo
bzr branch http://upstream local
^C
rm -rf local
bzr branch http://upstream local

Should only copy the extra bits that it needs.

As an aside, because of how things are copied, all of inventory.knit is downloaded again. It would be nice if it could download data from file knits in an order that in knew when inventory revisions had been finished. (download in revision order, rather than file-id order). But that has its own problems.