title: Custom git merge union strategy for a Changelog
body: <p>We have an <code>ONGOING.md</code> file where each developer adds item when pushing code.<br>
  It looks like :</p>
  
  <pre><code>### Added
  - item 1
  
  ### Changed
  - item 2
  </code></pre>
  
  <p>It happened all the time that lines got overwritten when pulling/pushing code so I added a <code>.gitattributes</code> file at the repo root :</p>
  
  <pre><code>ONGOING.md -text merge=union
  </code></pre>
  
  <p>I expected that every written line got kept after that, but that's not the case, overwrites still happen.</p>
  
  <p>What is the proper way to deal with this?</p>
  
  <p><strong>Edit:</strong></p>
  
  <p>OK, it just happened so I copy/paste content of my  terminal :</p>
  
  <pre><code>$ more fab/hotfix/ONGOING.md 
  ### Added
  
  $ nano fab/hotfix/ONGOING.md; git commit fab/hotfix/ONGOING.md -m "update ongoing"
  
  $ more fab/hotfix/ONGOING.md 
  ### Added
  - add slug column to BO fack topic  admin  page
  
  $ git pull
  remote: Enumerating objects: 14, done.
  remote: Counting objects: 100% (14/14), done.
  remote: Compressing objects: 100% (13/13), done.
  remote: Total 14 (delta 1), reused 0 (delta 0)
  Unpacking objects: 100% (14/14), done.
  From gitlab.com:kraymer/website
     a740fe8a0..12d531e8d  hotfix              -&gt; origin/hotfix
  First, rewinding head to replay your work on top of it...
  Applying: add slug column to BO fack topic  admin  page
  Using index info to reconstruct a base tree...
  M   fab/hotfix/ONGOING.md
  Falling back to patching base and 3-way merge...
  
  $ more fab/hotfix/ONGOING.md 
  ### Added
  - shared task for old notifications to be deleted
  </code></pre>
  
  <p>I thought the sentence <em>"Falling back to patching base and 3-way merge..."</em> meant git resolved a conflict, so the merge driver should come into play, no?</p>
  
  <p><strong>Edit2:</strong></p>
  
  <p>So quoting @VonC :</p>
  
  <blockquote>
    <p>But if the 3-way merge completes without conflicts... no merge driver called.</p>
  </blockquote>
  
  <p>So I guess my problem can be rephrased like : how can I configure git so a 3-way merge on <code>ONGOING.md</code> ends up with a conflict when 2 developers edit the same section (like <code>### Added</code> in my previous example), such that the merge driver comes into play ?</p>
  
  <p>If we reconsider my example in Edit1, I  don't get how git ends up picking up the other dev line rather than mine or both.</p>