update to 2.31.0-rc2
    
    This release includes a fix for CVE-2021-21300¹ in addition to the other
    changes along the path to the final 2.31.0 release.
    
    Release notes:
    https://github.com/git/git/raw/v2.31.0-rc2/Documentation/RelNotes/2.31.0.txt
    
    ¹ Per the 2.17.6 release notes on CVE-2021-21300:
        On case-insensitive file systems with support for symbolic links, if
        Git is configured globally to apply delay-capable clean/smudge
        filters (such as Git LFS), Git could be fooled into running remote
        code during a clone.