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