filemerge: stop passing around 3 unused `None` values in `files` argument Since 67cfffbfb6a0 (filemerge: eliminate most uses of tempfiles, 2017-08-31), we have used only the last entry in the `files` tuple that gets passed around to various functions. That commit said "Emphasize that they're unused so we can more easily remove them later.". It's now "later". Differential Revision: https://phab.mercurial-scm.org/D11877