Using branch: revspec in stat blows up

Using a 'branch:' revspec in a stat command will blow itself up with a
    ReadOnlyError: A write attempt was made in a read only transaction on LockableFiles(lock, file:///where/ever/.bzr/repository/)
(where the path refers to the 'local' branch)

Reproduce:

#!/bin/sh -x
bzr="/usr/local/bin/bzr --no-plugins --no-aliases"

# Create initial branch
${bzr} init A
(
    cd A ;
    touch a ;
    ${bzr} add ;
    ${bzr} ci -m 'add A'
)

# Branch it and make a change
${bzr} branch A B
(
    cd B ;
    touch b ;
    ${bzr} add ;
    ${bzr} ci -m 'add in B'
)

# Try and 'stat' those changes from A
(
    cd A ;
    # If we use ancestor: instead of branch:, this works. Also, if we do
    # a 'diff' instead of 'stat', it works just fine. It's only 'stat'
    # with 'branch' that croaks.
    ${bzr} stat -r-1..branch:../B
)