remote: add a failing test for checking the current branch's upstream

remote: add a failing test for checking the current branch's upstream

When we update FETCH_HEAD we check whether the remote is the current
branch's upstream remote. The code does not check whether the current
refspec is relevant for this reference but always tries to perform the
reverse transformation, which causes it to error out if the refspec
doesn't match the reference.

Thanks to Pierre-Olivier Latour for the reproduction recipe.