mergedBranchExposer.sh

#!/bin/bash -ex
sendSlackMsg(){
	curl -X POST --data-urlencode "payload={\"channel\": \"$1\", \"attachments\": [{\"title\":\"Merged branches report:\", \"text\":\"$2\",\"mrkdwn_in\":[\"text\"]}], \"icon_emoji\":\":shipit:\"}" https://hooks.slack.com/services/incoming-webhook-url
}
#user-defined
cd $REPO 
git fetch -a --prune
declare -a textArray
git branch -r --merged origin/master | \
awk '{branch=substr($1,8); if(branch!="HEAD" && branch!="master") {print branch}}' | {
	while read branch; do
		git fetch origin $branch
		author=$(git log origin/$branch --format="%an" -n 1)
		branchTxt='*'$branch'*, last committer: *'$author'*'
		textArray+=("$branchTxt")
	done
	printf '%s\n' "${textArray[@]}" > branchtmp.out
}

text=$(cat branchtmp.out)
rm branchtmp.out
sendSlackMsg "#engineering" "$text"

Explanation: http://mtyurt.net/2017/02/28/bash-script-to-notify-slack-channel-for-merged-branches/