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

simplemerge: remove code for checking binary input now that callers do it The callers now do the checking for binary inputs and handle warnings and/or errors, so we can remove that code from the low-level `simplemerge` module now. After this patch we just raise an error unless the caller told us to allow binary inputs. Differential Revision: https://phab.mercurial-scm.org/D12169

simplemerge: let filemerge check for binary inputs This is similar to the previous patch, but here we put a specialized copy of `simplemerge._verifytext()` in the the `filemerge` module instead. Differential Revision: https://phab.mercurial-scm.org/D12147

simplemerge: let extension check for binary inputs (unless `--text`) The `simplemerge` module does too much UI-related things. This patch puts a specialized copy of `simplemerge._verifytext()` in the simplemerge extension. Differential Revision: https://phab.mercurial-scm.org/D12146

simplemerge: store input data in MergeInput I want to be able to update the file contents for support for partial conflict resolution. Having the text available (and later settable) on `MergeInput` will enable that. Differential Revision: https://phab.mercurial-scm.org/D12144

simplemerge: convert MergeInput to regular, non-attr.ib class I'm about to add more logic and state to it. Differential Revision: https://phab.mercurial-scm.org/D12145