Git Rev Parse Windows Path

get_repo_root() returns OS-native separators on Windows

git rev-parse --show-toplevel always emits forward-slash paths, even on Windows. Terragrunt returned that string unchanged from get_repo_root(), so configurations that compared the result against path/filepath-style paths or fed it back into helpers expecting OS-native separators saw spurious mismatches and broken joins on Windows.

The output is now normalized to OS-native separators before being returned, so get_repo_root() produces C:\repo\path on Windows and /repo/path on Linux and macOS.

Reported in #5976.