merge: consider the file merged when using :merge-{local,other} Returning `None` from `simplemerge()` means that the caller interprets it as "no merge was necessary (because two sides matched)". See 6217040b2780 and issue2680 for some background. However, `simplemerge()` shouldn't even get called in such scenarios, and returning `None` means that the file is not considered merged, even though the contents actually were. See the affected test cases. Differential Revision: https://phab.mercurial-scm.org/D11999