resolve: also detect new :mergediff conflict markers The conflict markers created by `:mergediff` were not detected as conflicts, which affects both `commands.resolve.mark-check` and `mergetools.<tool>.check`. This patch fixes that. The new regex it uses for finding conflict markers is less restrictive because it `:mergediff` doesn't follow the `<<<<<<<` and `>>>>>>>` lines by a space (and a description). Hopefully lines like that don't give too many false positives. We can add back the space and make `:mergediff` add trailing spaces if it turns out to be a problem. OTOH, there will always be some false positives and we have ways of overriding the checks already. This patch can go onto the default or stable branch, depending on how much we care about an experimental feature. Differential Revision: https://phab.mercurial-scm.org/D9835