Fix tag an existed tag name of a repository

replace #8303 
When tag a image with a tag name which is existed in the repository,
docker should prompt it is conflicted and it will only worked when with
a -f option.But the docker doesn't work like that,it will just
replace the image whether you have a -f option or not.View the
code you can find that if an repoName is existed,it will never go
to check if 'force' option is set or not.
Ya you can pass out without fmt.sprint