NAK.  These must not show up in protocol advertisements, etc.  If an application needs them it should read them.

Besides both files are *not* standard ref format.  They can contain more than one ref.  And in FETCH_HEAD's case, they also have URL data and branch data.  The fact that they can work like a ref is a bug in the C Git implementation that only looks at the first 40 bytes of a "ref like file" to see if it is a SHA-1.  In hindsight we might never have permitted these files to be "ref like" by having a stricter ref file parser.