mergestate: update _stateextras instead of reassinging `merge.applyupdates()` can store extras for a file and _stateextras may not be empty always. Hence reassigning loses the old values. We can directly update like this because we switched to using `collections.defaultdict` for `_stateextras` sometime ago.

tests: add some debugmergestate calls in `test-merge-criss-cross.t` This will help notice the change in upcoming patches much better. Differential Revision: https://phab.mercurial-scm.org/D9028

tests: add few debugrevlogindex and a log call to see changes in next test In upcoming patches, we will create a new filenode in couple of cases. Let's add some debugrevlogindex and a log call so that we can observe those changes better. Differential Revision: https://phab.mercurial-scm.org/D8987

merge: disable `m2-vs-ma` optimization if new filenode config is true The `m2-vs-ma` optimization filters out the file which have not changed between second parent and the ancestor of the merge. This results in the m1-vs-m2 diff not processing those files. These files will be required when we are creating new filenode for salvaged cases as we need to track them to store in mergestate that file can be salvaged. Differential Revision: https://phab.mercurial-scm.org/D9027

configitems: add a new config option to control new filenode functionality This series is adding functionality where we can force create a new filenode for some files on a merge-commit. This is meant to represent an explicit choice made by user and hence distingusing the file in the merged commit with the file in parent by creating a new filenode instead of using the parent one. This introduces a experimental config option under which this functionality will be hidden. A new testcase is added in tests/test-merge-criss-cross.t to test the new functionality while making the option does not break other scenarios. Differential Revision: https://phab.mercurial-scm.org/D9026

git: add test showing `hg commit -i` working on a git repo Shows `hg commit -i` working on a git repo via the `git` extension. Adds working directory changes to files `alpha` and `beta`, then selects only `alpha` changes and commits them. As of now this would fail for a filename that includes uppercase characters due to the lack of case folding support in the extension. Differential Revision: https://phab.mercurial-scm.org/D9081

merge: replace calls to hg.updaterepo() by merge.update() The former no longer buys us anything. Differential Revision: https://phab.mercurial-scm.org/D9067

merge: add a higher-level update() for the common `hg update` use case This is different from the `update()` function that I just made private. The new function is specifically for the normal `hg update` use case. It doesn't do a merge and it doesn't do a clean (forced) update. Differential Revision: https://phab.mercurial-scm.org/D9066

sidedata: simply read p2copies files from the `ChangingFiles` object

sidedata: simply read p1copies files from the `ChangingFiles` object