branching: merge with stable

rust: Fix outdated comments in OwningDirstateMap OwningDirstateMap used to own a PyBytes, but was generalized to be more generic when it was moved from hg-cpython to hg-core. This fixes some comments that were still referencing PyBytes. Differential Revision: https://phab.mercurial-scm.org/D12003

merge: consider the file merged when using :merge-{local,other} Returning `None` from `simplemerge()` means that the caller interprets it as "no merge was necessary (because two sides matched)". See 6217040b2780 and issue2680 for some background. However, `simplemerge()` shouldn't even get called in such scenarios, and returning `None` means that the file is not considered merged, even though the contents actually were. See the affected test cases. Differential Revision: https://phab.mercurial-scm.org/D11999

simplemerge: delete unused exception class `CantReprocessAndShowBase` The only user was removed in f18830651811 (simplemerge: burn "minimal" feature to the ground, 2014-08-05). Differential Revision: https://phab.mercurial-scm.org/D11998

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