uninteresting=master # or some other way to identify ones to discard
git for-each-ref |
while read sha1 reftype refname; do
    if git merge-base --is-ancestor $refname $uninteresting; then
        # $refname / $sha1 "is merged", or is exactly equal
        ... do whatever you like here ...
    else
        # there is no ancestry path from $uninteresting back to $refname
        ... do whatever else you like here ...
    fi
done

git for-each-ref --no-merged master refs/heads/

--no-merged [<object>]:

* kn/for-each-tag-branch:
  for-each-ref: add '--contains' option
  ref-filter: implement '--contains' option
  parse-options.h: add macros for '--contains' option
  parse-option: rename parse_opt_with_commit()
  for-each-ref: add '--merged' and '--no-merged' options
  ref-filter: implement '--merged' and '--no-merged' options
  ref-filter: add parse_opt_merge_filter()
  for-each-ref: add '--points-at' option
  ref-filter: implement '--points-at' option