add '%d' pretty format specifier to show decoration

add '%d' pretty format specifier to show decoration

Add a new format placeholder, %d, which expands to a ref name decoration
(think git log --decorate).  It expands to an empty string if the commit
has no decoration, or otherwise to a comma (and space) separated list of
decorations, surrounded by parentheses and a leading space.

Michael Dressel implemented an initial version and chose the letter d,
Junio suggested to add a leading space and parentheses.

Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>