for-each-ref: add '--merged' and '--no-merged' options

for-each-ref: add '--merged' and '--no-merged' options

Add the '--merged' and '--no-merged' options provided by 'ref-filter'.
The '--merged' option lets the user to only list refs merged into the
named commit. The '--no-merged' option lets the user to only list refs
not merged into the named commit.

Add documentation and tests for the same.

Based-on-patch-by: Jeff King <peff@peff.net>
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>