builtin/notes: improve naming

builtin/notes: improve naming

In preparation for some needed refactoring, rename struct msg_arg to
struct note_data, and rename its instances from "msg" to "d" (also
removing some unnecessary parentheses). The 'msg_arg' name was
inherited from tag.c, but is not really a good name for the contents
of a note.

Also rename write_note_data() to copy_obj_to_fd(), which more aptly
describes what it actually does: Copying the contents of a git object
(given by its SHA1) into a given file descriptor.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>