merge: make applyupdates() not mutate mresult argument We have an extension at work that overrides `merge.applyupdates()` to make it skip some writes and instead instruct the virtual filesystem we use to get a different version. That override doesn't work correctly when doing `hg co -m` and there's a modified file in the dirstate that's deleted in the destination. That's because `applyupdates()` mutates its `mresult` argument and our extension had passed in a modified copied of `mresult` to the overridden function, which resulted in the mutation not having any effect. This patch fixes that by letting the caller (i.e. `merge._update()`) update `mresult` with the extra actions instead. Besides fixing our internal extension, that seems cleaner to me anyway (better to not mutate `mresult` only in some cases and we can skip some of the logic if we're not going to update the dirstate anyway). Differential Revision: https://phab.mercurial-scm.org/D10830

index: use an explicit constant for INDEX_HEADER format and use it for docket This avoid leaking python-3.6 compatibility details too much. Differential Revision: https://phab.mercurial-scm.org/D10831

verify: use some intermediate variables instead of a multi-liner This is shorter and easier to read as the indentation remains the same. Differential Revision: https://phab.mercurial-scm.org/D10822

verify: use some intermediate variables instead of a multi-liner This is shorter and easier to read as the indentation remains the same. We extract the long message in a module level constant for clarity. Differential Revision: https://phab.mercurial-scm.org/D10821

verify: use some intermediate variables instead of a multi-liner This is shorter and easier to read as the indentation remains the same. Differential Revision: https://phab.mercurial-scm.org/D10820

verify: use some intermediate variables instead of a multi-liner This is shorter and easier to read as the indentation remains the same. We extract the long message in a module level constant for clarity. Differential Revision: https://phab.mercurial-scm.org/D10819

verify: use some intermediate variables instead of a multi-liner This is shorter and easier to read as the indentation remains the same. Differential Revision: https://phab.mercurial-scm.org/D10818

verify: use some intermediate variables instead of a multi-liner This is shorter and easier to read as the indentation remains the same. Differential Revision: https://phab.mercurial-scm.org/D10817

verify: use some intermediate variables instead of a multi-liner This is shorter and easier to read as the indentation remains the same. Differential Revision: https://phab.mercurial-scm.org/D10816

verify: use some intermediate variables instead of a multi-liner This is shorter and easier to read as the indentation remains the same. We extract the long message in a module level constant for clarity. Differential Revision: https://phab.mercurial-scm.org/D10815