ref-filter: introduce handler function for each atom

ref-filter: introduce handler function for each atom

Introduce a handler function for each atom, which is called when the
atom is processed in show_ref_array_item().

In this context make append_atom() as the default handler function and
extract quote_formatting() out of append_atom(). Bump this to the top.

Mentored-by: Christian Couder <redacted>
Mentored-by: Matthieu Moy <redacted>
Signed-off-by: Karthik Nayak <redacted>
Signed-off-by: Junio C Hamano <redacted>