Update git-clone.md :`cf_export` should use `--mask` for creds

[takac]

Use --mask when handling credentials so they aren't printed into the build logs

[NimRegev]

That's fine; are we sure we don't need to update any where else? For example, in this one: https://codefresh.io/docs/docs/pipelines/variables/#exporting-variables-with-cf_export