title: git merge discards new changes without conflict
body: <p>I have really strange problem that happens sometimes in my team.<br>
  Let me describe it using the picture:</p>
  
  <p><a href="https://i.stack.imgur.com/zGYUk.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/zGYUk.png" alt="Commits"></a></p>
  
  <p>Let us say that we have two branches that have same base commit "commit 1".<br>
  Now, in "commit 2", someone added new line to file X.txt.<br>
  In the "commit 3", somebody added the line file Y.txt. </p>
  
  <p>After that there are some more commits, some more branches and finaly we are merging those two branches. Seams like totaly normal merge. </p>
  
  <p>But sometimes something strange happen because Git can decide that the change that was made in the green branch in the Y file should be discarded, even if there were totaly no changes of this file in red branch.<br>
  Sometimes it even delete files that was added to the green branch but not to red branch. </p>
  
  <p>I'm totally puzzled, I have no idea why this is happening.<br>
  We are using SourceTree, and project is in Unity.</p>
  
  <p>I will be gratefull for any ideas ;-).</p>