`Index.add()` accepts files behind symlinks

Hey.

Not sure whether this is a bug or considered valid (though probably strange) usage, but Index.add() - and presumably Index.add_all() as well as IndexEntry, too - accept paths where even non-final components are symbolic links, like in symlink-to-dir/some-non-dir-file.

git-add itself rejects these with an error like:

$ git add symlink-to-dir/some-non-dir-file
fatal: pathspec 'symlink-to-dir/some-non-dir-file' is beyond a symbolic link

The Index object even uses these pathnames and doesn't just resolve them:

list(r.index)
[<pygit2.index.IndexEntry path=symlink-to-dir/some-non-dir-file id=e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 mode=33188>]

Creating a commit of that works, too.

Now the problem with such pathnames is that symlink-to-dir is not added as a symbolic link to the index (at least not automatically), instead it seems to behave as if there were a true directory of that name which contained the file, but which is then immediately gone after adding.

IMO hat leads to all kinds of unexpected stuff (which is probably the reason why git-add forbids it instead of e.g either resolving it or doing something else (like resolving it and adding the file under its real dir).

Maybe pygit2 should also reject these? Not sure what libgit2 does, though.

Cheers,
Chris.

Comienza reproduciendo el comportamiento informado de Index.add() con un componente de symlink que no sea el último, y compáralo después con el rechazo de la misma ruta por parte de git-add. Comprueba si Index.add_all() e IndexEntry se comportan de forma similar y revisa el comportamiento correspondiente de libgit2. Se considera terminado cuando se haya establecido el comportamiento esperado y este esté cubierto por una prueba de regresión.