ref-filter: implement '--merged' and '--no-merged' options

ref-filter: implement '--merged' and '--no-merged' options

In 'branch -l' we have '--merged' option which only lists refs (branches)
merged into the named commit and '--no-merged' option which only lists
refs (branches) not merged into the named commit. Implement these two
options in ref-filter.{c,h} so that other commands can benefit from this.

Based-on-patch-by: Jeff King <redacted>
Mentored-by: Christian Couder <redacted>
Mentored-by: Matthieu Moy <redacted>
Signed-off-by: Karthik Nayak <redacted>
Signed-off-by: Junio C Hamano <redacted>