meta: changelog for v1.3.1

docs/changelog.md

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+v1.3.1
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+🔒 This is a security release to provide compatibility with git's changes to address [CVE 2022-24765](https://github.blog/2022-04-12-git-security-vulnerability-announced/).
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+**libgit2 is not directly affected** by this vulnerability, because libgit2 does not directly invoke any executable. But we are providing these changes as a security release for any users that use libgit2 for repository discovery and then _also_ use git on that repository. In this release, we will now validate that the user opening the repository is the same user that owns the on-disk repository. This is to match git's behavior.
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+In addition, we are providing several correctness fixes where invalid input can lead to a crash. These may prevent possible denial of service attacks. At this time there are not known exploits to these issues.
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+Full list of changes:
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+* Validate repository directory ownership (v1.3) by @ethomson in https://github.com/libgit2/libgit2/pull/6268
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+All users of the v1.3 release line are recommended to upgrade.
+
 v1.3
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