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