Fixing the return value of rename_variable() on Windows

[florisvdh]

Description

This PR adds some fixes related to the return value of rename_variable().

• In the documentation, the description of the return value was wrong (NULL).
• On Windows it has been observed that the root remains included in the result, despite remove_root() was called. During debugging on Windows, it was seen that Windows-specific pattern mismatches occurred between the root and file arguments of remove_root(). These could be "\\" vs. "/" as well as user directory abbreviations. Applying normalizePath() on both inputs of remove_root() before calling the latter, similar to the way this is handled in write_vc() (where the problem doesn't pop up), solved it.
• A test has been added to verify that the root is not part of the resulting file vector.

I'm pushing this in two steps, hoping to see failure of the tests in Windows first, then succes after adding the commit that fixes behaviour.

[florisvdh]

The force-push was to correct the commit message.

[florisvdh]

As hoped, the Windows check of d6ab4ad demonstrates the issue:

══ Failed tests ════════════════════════════════════════════════════════════════
── Failure ('test_g_rename_variable.R:249:3'): rename_variable() shows no root in returned file names ──
Expected every element of `result` not to match regexp "git2rdata-rename-".
Actual text:
✖ │ C:/Users/RUNNER~1/AppData/Local/Temp/RtmpE1UI4b/working_dir/RtmpCaFXlZ/git2rdata-rename-276027fb75c5/unsorted.tsv
✖ │ C:/Users/RUNNER~1/AppData/Local/Temp/RtmpE1UI4b/working_dir/RtmpCaFXlZ/git2rdata-rename-276027fb75c5/unsorted.yml

It is fixed by f486662.

[ThierryO]

Thanks for the PR. I'll fix the other issues raised by checklist.