[BUG] readonly operations interfere with each other

[Aaron Bentley]

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If you invoke "bzr log|less" and then "bzr diff -r 535|less", the diff
output will be blank until you reach the bottom in the log listing.  I
assume both operations lock the branch.  It would be handy if you could
execute two readonly operations at the branch.  The standard semantics:

1. the read lock can only be acquired when the write lock is not active.
~ The read lock can be acquired by several processes simultaneously, and
is reference-counted.
2. the write lock can only be acquired when the read lock is not active.
~ The write lock can only be acquired by one process at a time.

Aaron
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