fsck: warn about symlinked dotfiles we'll open with O_NOFOLLOW

In the commits merged in via 204333b015 (Merge branch 'jk/open-dotgitx-with-nofollow', 2021-03-22), we stopped following symbolic links for .gitattributes, .gitignore, and .mailmap files. Let's teach fsck to warn that these symlinks are not going to do anything. Note that this is just a warning, and won't block the objects via transfer.fsckObjects, since there are reported to be cases of this in the wild (and even once fixed, they will continue to exist in the commit history of those projects, but are not particularly dangerous). Note that we won't add these to the existing gitmodules block in the fsck code. The logic for gitmodules is a bit more complicated, as we also check the content of non-symlink instances we find. But for these new files, there is no content check; we're just looking at the name and mode of the tree entry (and we can avoid even the complicated name checks in the common case that the mode doesn't indicate a symlink). We can reuse the test helper function we defined for .gitmodules, though (it needs some slight adjustments for the fsck error code, and because we don't block these symlinks via verify_path()). Note that I didn't explicitly test the transfer.fsckObjects case here (nor does the existing .gitmodules test that it blocks a push). The translation of fsck severities to outcomes is covered in general in t5504. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>