WIP.

.pre-commit-config.yaml

+-   repo: https://github.com/pre-commit/pre-commit-hooks
+    sha: v0.7.1
+    hooks:
+    -   id: check-added-large-files
+    -   id: check-merge-conflict
+    -   id: check-yaml
+-   repo: https://www.shore.co.il/git/shell-pre-commit/
+    sha: v0.5.4
+    hooks:
+    -   id: shell-lint
+        files: &shellscripts 'merge-conflict'
+    -   id: shellcheck
+        files: *shellscripts
+-   repo: ./
+    sha: HEAD
+    hooks:
+    - id: merge-conflict

.pre-commit-hooks.yaml

+---
+- id: merge-conflict
+  name: merge conflicts
+  description: Checks for merge conflicts to the master branch.
+  language: script
+  entry: merge-conflict
+  files: '$^'
+  always_run: True

.travis.yml

+---
+language: python
+python: "3.6"
+dist: trusty
+sudo: false
+group: beta
+cache:
+  - pip
+  - directories:
+      - $HOME/.pre-commit
+      - $HOME/.cabal
+      - $HOME/.ghc
+
+addons:
+  apt:
+    packages:
+      - cabal-install
+      - ghc
+
+env:
+    PATH: "$HOME/bats/bin:$HOME/.cabal/bin:$PATH"
+
+install:
+  - cabal update && cabal install shellcheck
+  - pip install pre_commit | cat
+  - git clone --depth 1 https://github.com/sstephenson/bats.git "$HOME/bats"
+
+script:
+  - pre-commit run --all-files
+
+notifications:
+  on_failure: never
+  email: false

hooks.yaml

+.pre-commit-hooks.yaml
\ No newline at end of file

merge-conflict

+#!/bin/sh
+set -eu
+
+dest="${1:-master}"
+current="$(git symbolic-ref --short HEAD)"
+
+[ "$current" != "$dest" ] || exit
+
+git format-patch "$(git merge-base HEAD "$dest")..$dest" --stdout | git apply --check -

tests/merge-conflict.bats

+#!/usr/bin/env bats
+
+export PATH="$BATS_TEST_DIRNAME/../:$PATH"
+export repo="$BATS_TMPDIR/testrepo"
+
+@test "setup" {
+    if [ ! -f "$repo/file" ]
+    then
+        git init "$repo"
+        cd "$repo"
+        echo 1 > file
+        git -C "$repo" add "$repo/file"
+        git -C "$repo" commit -m "Initial commit."
+        git -C "$repo" checkout -b master2
+        git -C "$repo" checkout -b no-conflict
+        git -C "$repo" checkout -b conflict
+        echo 2 > "$repo/file"
+        git -C "$repo" commit file -m "Conflict with master."
+        git -C "$repo" checkout master
+        echo 3 > "$repo/file"
+        git -C "$repo" commit file -m "Conflict with the conflict branch."
+    fi
+}
+
+@test "Same branch" {
+    cd "$repo"
+    git checkout master || true
+    merge-conflict
+}
+
+@test "No conflict" {
+    cd "$repo"
+    git checkout no-conflict || true
+    merge-conflict
+}
+
+@test "Different than master" {
+    cd "$repo"
+    git checkout master || true
+    merge-conflict master2
+}
+
+@test "Conflict" {
+    cd "$repo"
+    git checkout conflict || true
+    ! merge-conflict
+}
+
+@test "teardown" {
+    rm -rf "$repo"
+}