go-git: Malicious reference names may modify files outside the reference storage

go-git is an extensible git implementation library written in pure Go. Prior to 5.19.2 and 6.0.0-alpha.5, reference names are not sanitized before being used to construct on-disk paths under the reference storage directory, so a maliciously crafted reference name (for example containing directory-traversal sequences) can cause go-git to write files outside the intended reference storage directory. Versions 5.19.2 and 6.0.0-alpha.5 fix the issue.