branching from an -in-use tree errors

[Robert Collins]

bzr branch localtree new-branch, while 'bzr pull' was running in
localtree:

bzr: ERROR: Could not acquire lock "[Errno 11] Resource temporarily
unavailable"
UserWarning: lock on <open file u'...localtree/.bzr/checkout/dirstate',
mode 'rb' at 0x15f5468> not released warn("lock on %r not released" %
self.f)

This is presumably due to the accelerated 'copy-content-from-tree' code
path; it would be nice to handle this and fall back to just opening the
branch.

the relevant parts of the bt:
File "/home/robertc/source/baz/bzr-test-fixes/bzrlib/bzrdir.py", line
1451, in create_workingtree
accelerator_tree=accelerator_tree, hardlink=hardlink)
File "/home/robertc/source/baz/bzr-test-fixes/bzrlib/workingtree_4.py",
line 1366, in initialize
basis = accelerator_tree.revision_tree(revision_id)
File "/home/robertc/source/baz/bzr-test-fixes/bzrlib/decorators.py",
line 136, in read_locked
self.lock_read()

 affects bzr

--
GPG key available at: <http://www.robertcollins.net/keys.txt>.