Fixed active_refspecs field not initialized on new git_remote objects

This is a bug fix resulting from the investigation of #2670. I simply copy-pasted the piece of code from the implementation of `git_remote_load()`.

When creating a new remote, contrary to loading one from disk, `active_refspecs` is not populated. This means that if using the new remote to push, `git_remote_update_tips()` will be a no-op since it checks the refspecs passed during the push against the base ones i.e. `active_refspecs`. And therefore the local refs won't be created or updated after the push operation.

All the unit tests pass.
I copy pasted from another test code in the page IIRC.