simplemerge: move printing of merge result to extension The `mercurial.simplemerge` module still has some command-lines processing that doesn't belong in such a low-level module. This patch moves the handling of `hg simplemerge --print` to the extension by having `mercurial.simplemerge.simplemerge()` return the merged text. Differential Revision: https://phab.mercurial-scm.org/D12148

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