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

rust: upgrade `rand*` crates `test-check-cargo-lock.t` is failing for me and I was hoping this would help. It doesn't, but we might as well take the upgrade now that I've done the (small amount of) work for it. Differential Revision: https://phab.mercurial-scm.org/D12000

logcmdutil: raise `StateError` when file to follow doesn't exist Differential Revision: https://phab.mercurial-scm.org/D11969