tests: use new, use-case-specific methods from merge module Differential Revision: https://phab.mercurial-scm.org/D8169

merge: introduce a merge() for that use-case In the same vein as some earlier patches like f546d2170b0f (merge: introduce a clean_update() for that use-case, 2020-01-15). Differential Revision: https://phab.mercurial-scm.org/D8168

merge: drop redundant mergeforce argument from hg.merge() The only caller that passed a value for either `force` or `mergeforce` passed the same value for both, so let's simplify the interface by accepting only `force`. Differential Revision: https://phab.mercurial-scm.org/D8167

merge: change default of hg.merge()'s "force" argument from None to False The argument is only passed to `mergemode.update()`, and that function treats `None` just like `False`, so `False` seems clearer. Differential Revision: https://phab.mercurial-scm.org/D8166