`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.

Beginne damit, das gemeldete Verhalten von Index.add() mit einer nicht letzten Symlink-Komponente zu reproduzieren, und vergleiche es anschließend mit der Ablehnung desselben Pfads durch git-add. Prüfe, ob sich Index.add_all() und IndexEntry ähnlich verhalten, und untersuche das entsprechende Verhalten von libgit2. Als abgeschlossen gilt die Aufgabe, wenn die erwartete Behandlung festgestellt und durch einen Regressionstest abgedeckt ist.