This utilizes an external text file (branches.txt) and runs a test against the branches to see if there is a merge conflict.

git_merge_check.sh

#!/bin/bash

function git_merge_test() {
    echo "test git branch merges";
    RUN_UNIT_TESTS=0

    while test $# -gt 0; do
            case "$1" in
                -h|--help)
                    echo "test the branches in branches.txt for merge conflicts"
                    echo " "
                    echo "git_merge_test [options]"
                    echo " "
                    echo "options:"
                    echo "-h, --help            show brief help"
                    echo "-t, --test            execute unit tests on successful merge"
                    return 0
                    ;;
                -t)
                    shift
                    RUN_UNIT_TESTS=1
                    ;;
                *)
                    break
                    ;;
            esac
    done

    # prune, pull and update develop
    git prune && git checkout develop && git pull;

    # create test merge branch
    test_branch_name="test-merge$(date '+%d%m%S')";

    echo "Creating local testing branch $test_branch_name";

    git checkout -b $test_branch_name;

    branches="branches.txt";

    while IFS='' read -r branch || [[ -n "$branch" ]]; do
        # strip out any hash comments
        clean_branch=${branch%#*}
        # trim whitepsace
        trim_branch="$(echo -e "${clean_branch}" | tr -d '[:space:]')"
        # only execute if there is a branch
        if [ -n "$trim_branch" ]; then
            echo "Merging $trim_branch"
            (git merge --no-edit refs/remotes/origin/${trim_branch} && echo "$trim_branch: SUCCESS" >> build-result.txt) || (git merge --abort && echo "$trim_branch: FAIL" >> build-result.txt)
        fi;
    done < "$branches"

    cat build-result.txt

    if grep -F " FAIL" build-result.txt; then
        echo "Stopping because of a merge conflict in one of the branches";
        rm build-result.txt;
        # switch back to develop and delete the test branch
        git checkout develop;
        git reset --hard origin/develop;
        git branch -D $test_branch_name;
        return 1;
    fi

    rm build-result.txt;

    if [ $RUN_UNIT_TESTS == 1 ]; then
        echo "Executing build and unit tests...";
        grunt;
    fi;

    # switch back to develop and delete the test branch
    git checkout develop;
    git reset --hard origin/develop;
    git branch -D $test_branch_name;
};

No, I haven't dug into finding a way to grab the messages git responds with to more accurately tell you what "SUCCESS" or "FAILURE" means in a "the branch was already merged" kind of way.

Made the unit test execution a passed in argument for now - there's a few instances where I just want to test the branch merges and not the tests.