The dummy negotiation scheme is used for known authentication strategies
that do not wish to act.  For example, when a server requests the
"Negotiate" scheme but libgit2 is not built with Negotiate support, and
will use the "dummy" strategy which will simply not act.

Instead of setting `out` to NULL and returning a successful code, return
`GIT_PASSTHROUGH` to indicate that it did not act and catch that error
code.