The git-scm website says that it's at github under the download section.
However Wikipedia says it's at git.kernel.org. There's also corresponding clone links in that site.
1 Answers
https://git.kernel.org is correct, specifically https://git.kernel.org/pub/scm/git/git.git/.
https://github.com/git/git has "About" text that indicates it is a mirror:
Git Source Code Mirror - This is a publish-only repository but pull requests can be turned into patches to the mailing list via GitGitGadget (https://gitgitgadget.github.io/).
git-scm.com seems to be only semi-official: It's linked from the source tree's README but per the About this site link at the bottom, git-scm is an open-source third-party piece of documentation that became the Pro Git book (advertised in the left column). Since the official repo is based on cgit and is relatively minimal in its UI, I imagine GitHub and git-scm are more welcoming to newcomers and thus more popular.
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