debugmergestate: make templated Our IntelliJ team wants to be able to read the merge state in order to help the user resolve merge conflicts. They had so far been reading file contents from p1() and p2() and their merge base. That is not ideal for several reasons (merge base is not necessarily the "graft base", renames are not handled, commands like `hg update -m` is not handled). It will get especially bad as of my D7827. This patch makes the output s a templated. I haven't bothered to make it complete (e.g. merge driver states are not handled), but it's probably good enough as a start. I've done a web search for "debugmergestate" and I can't find any indication that any tools currently rely on its output. If it turns out that we get bug reports for it once this is released, I won't object to backing this patch out on the stable branch (and then perhaps replace it by a separate command, or put it behind a new flag). Differential Revision: https://phab.mercurial-scm.org/D8113