gn build: Use "git rev-parse --git-dir" to discover the path to the .git directory.

This makes it compatible with worktrees.

Differential Revision: https://reviews.llvm.org/D56422

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350897 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed