The Single UNIX Specification mentions the "--" convention in 1997 (at which point it was already in widespread use): https://pubs.opengroup.org/onlinepubs/7908799/xbd/utilconv.h...

The first release of git was in 2005, and Torvalds' Linux was clearly UNIX-inspired, so it's not like this was due to considerations for some other OS like DOS/Windows.