odb: fix subtle logic to check whether an alternate is usable

When adding an alternate to the object database we first check whether or not the path is usable. A path is usable if: - It actually exists. - We don't have it in our object sources yet. While the former check is trivial enough, the latter part is somewhat subtle and prone for bugs. This is because the function doesn't only check whether or not the given path is usable. But if it _is_ usable, we also store that path in the map of object sources immediately. The tricky part here is that the path that gets stored in the map is _not_ copied. Instead, we rely on the fact that subsequent code uses `strbuf_detach()` to store the exact same allocated memory in the created object source. Consequently, the memory is owned by the source but _also_ stored in the map. This subtlety is easy to miss, so if one decides to refactor this code one can easily end up breaking this mechanism. Make the relationship more explicit by not storing the path as part of `alt_odb_usable()`. Instead, store the path after we have created the source so that we can use the source's path pointer directly. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>