Merge branch 'jc/tag-idempotent-no-op' into seen

"git tag T O" when the tag T is already pointing at the object O is
a no-op; we used to but no longer error out such a request and
require "--force" and instead turn it into a no-op.

Comments?

* jc/tag-idempotent-no-op:
  tag: allow idempotent "git tag" without "--force"

diff --cc builtin/tag.c
Simple merge