repository: set error message if trying to set HEAD to a checked out one

If trying to set the HEAD of a repository to another reference, we have
to check whether this reference is already checked out in another linked
work tree. If it is, we will refuse setting the HEAD and return an
error, but do not set a meaningful error message. Add one.

src/repository.c

@@ -2562,6 +2562,8 @@ int git_repository_set_head(
 
 	if (ref && current->type == GIT_REF_SYMBOLIC && git__strcmp(current->target.symbolic, ref->name) &&
 	    git_reference_is_branch(ref) && git_branch_is_checked_out(ref)) {
+		giterr_set(GITERR_REPOSITORY, "cannot set HEAD to reference '%s' as it is the current HEAD "
+			"of a linked repository.", git_reference_name(ref));
 		error = -1;
 		goto cleanup;
 	}