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

edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true