ref-filter: allow merged and no-merged filters

ref-filter: allow merged and no-merged filters

Enable ref-filter to process multiple merged and no-merged filters, and
extend functionality to git branch, git tag and git for-each-ref. This
provides an easy way to check for branches that are "graduation
candidates:"

$ git branch --no-merged master --merged next

If passed more than one merged (or more than one no-merged) filter, refs
must be reachable from any one of the merged commits, and reachable from
none of the no-merged commits.

Signed-off-by: Aaron Lipman <alipman88@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>