git: skeleton of a new extension to _directly_ operate on git repos This is based in part of work I did years ago in hgit, but it's mostly new code since I'm using pygit2 instead of dulwich and the hg storage interfaces have improved. Some cleanup of old hgit code by Pulkit, which I greatly appreciate. test-git-interop.t does not cover a whole lot of cases, but it passes. It includes status, diff, making a new commit, and `hg annotate` working on the git repository. This is _not_ (yet) production quality code: this is an experiment. Known technical debt lurking in this implementation: * Writing bookmarks just totally ignores transactions. * The way progress is threaded down into the gitstore is awful. * Ideally we'd find a way to incrementally reindex DAGs. I'm not sure how to do that efficiently, so we might need a "known only fast-forwards" mode on the DAG indexer for use on `hg commit` and friends. * We don't even _try_ to do anything reasonable for `hg pull` or `hg push`. * Mercurial need an interface for the changelog type. Tests currently require git 2.24 as far as I'm aware: `git status` has some changed output that I didn't try and handle in a compatible way. This patch has produced some interesting cleanups, most recently on the manifest type. I expect continuing down this road will produce other meritorious cleanups throughout our code. Differential Revision: https://phab.mercurial-scm.org/D6734

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Octopus Merge Support

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=====================

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This will be moderately complicated, as we'll need to synthesize phony

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changeset entries to explode the octopus into "revisions" that only

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have two parents each. For today, we can probably just do something like

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    aaaaaaaaaaaaaaaaaaXX{20 bytes of exploded node's hex sha}

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where XX is a counter (so we could have as many as 255 parents in a

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git commit - more than I think we'd ever see.) That means that we can

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install some check in this extension to disallow checking out or

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otherwise interacting with the `aaaaaaaaaaaaaaaaaa` revisions.

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Interface Creation

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====================

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We at least need an interface definition for `changelog` in core that

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this extension can satisfy, and again for `basicstore`.

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Reason About Locking

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====================

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We should spend some time thinking hard about locking, especially on

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.git/index etc. We're probably adequately locking the _git_

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repository, but may not have enough locking correctness in places

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where hg does locking that git isn't aware of (notably the working

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copy, which I believe Git does not lock.)

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Clean up requirements

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=====================

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Right now (for historical reasons, mainly) hgext.git uses a

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.hg/this-is-git file to detect repositories that should be treated as

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git. We should look in the .hg/requires for the "git" requirement

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instead (we already set this requirement, so it's mostly keying off

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that instead of using an empty file.)