[llvm] [gn build] Use shutil.which to find git in write_vcsrevision.py (PR #142570)

[David Spickett via llvm-commits]

DavidSpickett wrote:

This is low risk and I'm sure a bot will complain eventually if there's a problem, so I'm going to land this.

https://github.com/llvm/llvm-project/pull/142570