simplemerge: use same newline detection for :mergediff as for :merge[3] The `:merge` and `:merge3` markers used the newline style detected from the base input. The `:mergediff` style always used LF as newline character. This patch teaches `:mergediff` to use the same detection as the others did. Differential Revision: https://phab.mercurial-scm.org/D11976