+Adds a 'tag' reference in .git/refs/tags/
+
+Unless `-f` is given, the tag must not yet exist in
+`.git/refs/tags/` directory.
+
+If one of `-a`, `-s`, or `-u <key-id>` is passed, the command
+creates a 'tag' object, and requires the tag message. Unless
+`-m <msg>` is given, an editor is started for the user to type
+in the tag message.