simplemerge: change _minimize() to minimize a single conflict `_minimize()` is weird in that it changes `a_lines` and `b_lines` but leaves `base_lines` unchanged, which means they'll be inconsistent. That was fine because the caller never used `base_lines`. With the recent refactorings of this code, we can now make this function cleaner by having it minimize a single conflict and not care about `base_lines`. This also makes the code simpler and makes the code for each `render_*()` function more similar. Differential Revision: https://phab.mercurial-scm.org/D11981

simplemerge: make minimize() a free function IMO, `Merge3Text` should be about merging text without knowing about conflict markers. "Conflict minimization" is about making conflict markers smaller, so it should be moved out. This patch does that. I'll refactor it next. Differential Revision: https://phab.mercurial-scm.org/D11980

simplemerge: simplify and rename `render_markers()` `render_markers()` now always renders minimized 2-way markers, so let's simplify and rename it accordingly. Differential Revision: https://phab.mercurial-scm.org/D11979

simplemerge: split out function for rendering :merge3 conflict markers The results in some duplicate, but the code is very straight-forward and I think it's worth it to have each conflict marker style in a separate function. Differential Revision: https://phab.mercurial-scm.org/D11978

simplemerge: move default labels to simplemerge extension I want to remove knowledge of paths from the `simplemerge` core module. As part of that, let's move the default labels from the core module to the extension. Differential Revision: https://phab.mercurial-scm.org/D11977

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

simplemerge: clarify names of functions that render conflict markers Differential Revision: https://phab.mercurial-scm.org/D11975

simplemerge: make merge_lines() a free function IMO, the rendering of a conflict as text with merge markers should be separate from the code for resolving conflicts. The latter is what `Merge3Text` is mostly about already. Differential Revision: https://phab.mercurial-scm.org/D11974

simplemerge: convert `merge_lines()` away from generator We always consume all the lines and put them in a list anyway. By making the function not a generator, we can later make it return an additional value (to indicate if there were conflicts). Differential Revision: https://phab.mercurial-scm.org/D11973

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