git: fix CVE-2021-21300

checkout: fix bug that makes checkout follow symlinks in leading path

Upstream-Status: Acepted [https://github.com/git/git/commit/684dd4c2b414bcf648505e74498a608f28de4592

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CVE: CVE-2021-21300
Signed-off-by: Minjae Kim <flowergom@gmail.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>