ref-filter: add support for %(upstream:track,nobracket)

ref-filter: add support for %(upstream:track,nobracket)

Add support for %(upstream:track,nobracket) which will print the
tracking information without the brackets (i.e. "ahead N, behind M").
This is needed when we port branch.c to use ref-filter's printing APIs.

Add test and documentation for the same.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>