dirstate: add a function to update tracking status while "moving" parents
The `scmutil.dirstateparent` is moving the dirstate parent without touching the
working copy. It is used by history-rewriting operations like amending of
folding.
The function was directly doing the "low level" computation and dirstate
change. All that logic belong to the dirstate and should be moved there.
For this purpose we introduce a new function that does just that and use it.
Differential Revision: https://phab.mercurial-scm.org/D11012
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