rebase: remove some redundant setting of dirstate parents
Since we're now setting the dirstate parents to its correct values
from the beginning (right after `merge.update()`), we usually don't
need to set them again before committing. The only case we need to
care about is when committing collapsed commits. So we can remove the
`setparents()` calls just before committing and add one only for the
collapse case.
Differential Revision: https://phab.mercurial-scm.org/D7828
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