Martin [off-list ref] writes:

And yes, for the documentation, it *should* be clear that, removing a
branch, removes the
commits on it.
But then it must be said, that the branch is first removed. That is
not currently the case.

Sorry, but I still do not see how it makes any difference if the
branch is first removed and then made to point at somewhere else, or
the branch gets just moved without any explicit or impolicit
removal.  A branch cannot point at two different commits at the same
time, so the end result is that the commit at the old tip is no
longer pointed at by the branch after the update.  In other words,

	----o---o---X---Y---Z

if it were possible to move the tip of a branch, that used to point
at commit Z, so that it points at commit X in the above picture,
without making it *not* to point at Z, then I would understand your
explanation, but I do not see how it would be possible.