[bug] unlock branch if we fail to lock repo

[John Arbash Meinel]

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The current bzr code gives this error if you ask to update a branch
while another branch is being updated:

bzr: ERROR: Could not acquire lock
LockDir(file:///path/.bzr/repository/lock)

H:\dev\bzr\bzr.dev\bzrlib\lockable_files.py:110: UserWarning: file group
LockableFiles(<bzrlib.transport.local.LocalTransport
url=file:///path/.bzr/branch/>) was not explicitly unlocked
  warn("file group %r was not explicitly unlocked" % self)


The reason is that locking the branch control files is succeeding, but
locking the repository is failing, and when the exception is raised, we
aren't doing any cleanup.

We are doing the right thing in WorkingTree.lock_write(), so I propose
we do the same in Branch.lock_write() with the following code:

def lock_write(self):
    """Lock both the branch and repository"""
    self.control_files.lock_write()
    try:
        self.repository.lock_write()
    except:
        self.control_files.unlock()
        raise

It is a small thing, but I think it gives better errors.

Should we be doing the same thing for 'lock_read()' even though it isn't
really locking anything?

John
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