[MERGE] various tweaks to push

[Robert Collins]

This is a few tweaks all conflated, because the issues weren't clear.

Basically - we were churning locks a lot (known), and code depended on
the locks being released for some methods (bad). 

So this patch changes branch.lock to only lock the repository once,
which means we can useful lock fallback repositories added to the branch
in set_stacked_on. The complexity with control_files is to deal with
AllInOne formats.

Anyhow, more reductions++.

-Rob
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