go-git: Improper parsing of specially crafted objects may lead to inconsistent interpretation compared to upstream Git

go-git is an extensible git implementation library written in pure Go. Prior to 5.19.0 and 6.0.0-alpha.3, go-git may parse malformed Git objects in a way that differs from upstream Git. When commit or tag objects contain ambiguous or malformed headers, go-git’s decoded representation may expose values differently from how Git itself would interpret or reject the same object. Additionally, go-git’s commit signing and verification logic operates over commit data reconstructed from go-git’s parsed representation rather than the original raw object bytes. As a result, go-git may sign or verify a commit payload that is not byte-for-byte equivalent to the object stored in the repository. This can cause a signature to appear valid for a commit whose displayed or effective metadata differs from the object that was intended to be signed. This vulnerability is fixed in 5.19.0 and 6.0.0-alpha.3.