simplemerge: use 3-way markers if mode=='merge3', ignoring number of labels Before this patch, we use a combination of `mode` and `labels` to decide which marker style to use. That's mostly my own fault because I was the one who added the `mergediff` as a "mode" instead of adding a separate argument for marker style. Still, let's make these arguments more orthogonal by ignoring the number of labels when deciding which marker style to use. This refactoring makes it a little clearer in the simplemerge *extension* that passing more than 2 `--label` arguments changes the marker style. Differential Revision: https://phab.mercurial-scm.org/D11972