Document possible code execution and integer overflow issue in git

PR: 208074
Reported by: Sevan Janiyan <venture37@geeklan.co.uk> (via PR)
Reported by: Tony Tung <tonytung@merly.org> (via email)
Security: CVE-2016-2315
Security: CVE-2016-2324
Security: https://vuxml.FreeBSD.org/freebsd/93ee802e-ebde-11e5-92ce-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/d2a84feb-ebe0-11e5-92ce-002590263bf5.html