[MERGE] Fix BzrDir.open in non-main (and non-server-request) thread when bzrlib.smart.request's _pre_open_hook is installed.

[Andrew Bennetts]

This fixes the bug Vincent found in bzrlib.smart.request's _pre_open_hook.
It makes it robust even in a new thread (which therefore has a pristine
thread-local jail_info), and adds a simple test to verify that this is so.

-Andrew.

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