Add MR conflict hook to Lefthook

.lefthook/pre-push/merge_conflicts

+#!/bin/zsh
+
+# Adjusted from https://gitlab.com/fdegier/pre-push-hooks with hardcoded values for speed
+ORIGIN=origin
+DEFAULT_BRANCH=master
+
+if [[ -n "$ORIGIN" ]]
+then
+  # Pull the default branch from remote
+  git fetch --quiet origin "$DEFAULT_BRANCH":"$DEFAULT_BRANCH"
+fi
+
+# Check for merge conflicts and abort
+if git merge --autostash --no-commit --no-ff --no-edit "$DEFAULT_BRANCH" > /dev/null 2>&1
+then
+  # Able to merge without conflicts
+  git merge --abort > /dev/null 2>&1
+  exit 0
+else
+  echo "Merge conflicts detected when merging to $DEFAULT_BRANCH!"
+  git merge --abort > /dev/null 2>&1
+  exit 1
+fi

lefthook.yml

@@ -86,6 +86,9 @@ pre-push:
       files: git diff --name-only --diff-filter=d $(git merge-base origin/master HEAD)..HEAD
       glob: 'data/removals/*.yml'
       run: echo "Changes to removals files detected. Checking removals..\n"; bundle exec rake gitlab:docs:check_removals
+  scripts:
+    "merge_conflicts":
+      runner: bash
 pre-commit:
   parallel: true
   commands: