Allow repo-root CWD for nbgv get-commits
Allow repo-root CWD for nbgv get-commits

Fixes #204 by ensuring that `GetSpecifiedOrCurrentDirectoryPath` always
has a trailing directory separator, like `Repository.Info.WorkingDirectory` does.

AArnott/Nerdbank.GitVersioning,AArnott/Nerdbank.GitVersioning,AArnott/Nerdbank.GitVersioning