Fixed git_remote_write_fetchhead() to handle tag refspecs

This should fix the 2nd part of #2674.

When passed a `refs/tags/` type of refspec, `git_remote_write_fetchhead()` was attempting to call `git_refspec_rtransform()` on it in order to figure out merge information for the FETCH_HEAD file.
This is possibly not a typo - "iff" is sometimes used to mean "if and only if".