builtin/notes.c: Split notes ref DWIMmery into a separate function

builtin/notes.c: Split notes ref DWIMmery into a separate function

expand_notes_ref() is a new function that performs the DWIM transformation
of "foo" -> "refs/notes/foo" where notes refs are expected.

This is done in preparation for future patches which will also need this
DWIM functionality.

Signed-off-by: Johan Herland <redacted>
Signed-off-by: Junio C Hamano <redacted>