Skips symlinks when walking files in a dir

[dcarney]

When walking a path (say, a large vendor/ directory in a Go repo), it's common to encounter symlinks. Attempting to stat/read these symlinks leads to spurious errors like the following:

2018/12/19 18:15:19 Unable to stat "vendor/github.com/grpc-ecosystem/go-grpc-middleware/auth/README.md": stat vendor/github.com/grpc-ecosystem/go-grpc-middleware/auth/README.md: no such file or directory
2018/12/19 18:15:19 Unable to stat "vendor/github.com/grpc-ecosystem/go-grpc-middleware/logging/README.md": stat vendor/github.com/grpc-ecosystem/go-grpc-middleware/logging/README.md: no such file or directory

This PR adds a simple check to see if a file is a symlink before writing it to the channel of files to check.