save_seqfile_data fails with Windows paths

[ressy]

Within save_seqfile_data, remove_shared_root_dir calls normalizePath on path strings but then assumes / as a path separator. On Windows normalizePath uses \ as the separator, and so the returned paths from remove_shared_root_dir are garbled. This causes save_seqfile_data to fail. This is a regression in 0.2.0 from 03df4fa.

I think we may just be able to give winslash="/" in normalizePath and keep the rest the same. .Platform$file.sep still returns "/" on Windows so apparently we can't just use that value instead of a literal /.

[ressy]

Manual test using 79cebf1 works now on Windows 10.