ref: check whether the target of the symref is a ref

Ideally, we want to the users use "git symbolic-ref" to create symrefs instead of writing raw contents into the filesystem. However, "git symbolic-ref" is strict with the refname but not strict with the referent. For example, we can make the "referent" located at the "$(gitdir)/logs/aaa" and manually write the content into this where we can still successfully parse this symref by using "git rev-parse". $ git init repo && cd repo && git commit --allow-empty -mx $ git symbolic-ref refs/heads/test logs/aaa $ echo $(git rev-parse HEAD) > .git/logs/aaa $ git rev-parse test We may need to add some restrictions for "referent" parameter when using "git symbolic-ref" to create symrefs because ideally all the nonpseudo-refs should be located under the "refs" directory and we may tighten this in the future. In order to tell the user we may tighten the above situation, create a new fsck message "symrefTargetIsNotARef" to notify the user that this may become an error in the future. Mentored-by: Patrick Steinhardt <ps@pks.im> Mentored-by: Karthik Nayak <karthik.188@gmail.com> Signed-off-by: shejialuo <shejialuo@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>