Fixed git_remote_write_fetchhead() to handle tag refspecs

If the remote mode is set to GIT_REMOTE_DOWNLOAD_TAGS_ALL, then calling
update_tips_for_spec() will call git_remote_write_fetchhead() passing a
refs/tags/ type of refspec. This then fails because git_refspec_rtransform()
gets called to figure out merge information for the FETCH_HEAD file, which
doesn't make sense for tags refspecs. The same problem also happens if
passing explicitly tag refspecs to git_remote_download().

The fix is simply to not attempt to figure out merge information in the
first place when handling tag refspecs.