split brain cache: Add a dummy ref update when necessary

git-update-ref does not make a new reflog entry if the ref is not
actually being changed.

This can mean that `dgit build' might fail to find a good cache entry,
but also fail to generate one that push will accept.

In this case, generate a dummy reflog update.  We make a spurious
child of the $dgitview, as that's straightforward.  Giving it the same
tree and a parent means that human use of reflog commands will produce
empty results rather than lots of confusing output.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

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