[MERGE][#250418] make bzrdir.clone_on_transport pass on stacked_on_pwd

[Michael Hudson]

Attempting a stacking-preserving clone on a branch that stacked on
another branch by a relative URL didn't work (bug 250418 on Launchpad,
"bzr_dir_for_branch_stacked_on_relative_url.clone('...',
preserve_stacking=True) miscalculates stacked-on URL").  The attached
one-line fix and 50-odd lines of test changes fixes this.  The bundle
includes and depends on my change to rebase_url posted a couple of hours
ago.

Cheers,
mwh
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