Copying codes from diff has an extra leading space

Summary

If I go to merge request copying codes like this:

And paste it somewhere, I ended up with an extra leading space:

       &.old {
         &::before {
           content: '-';
           position: absolute;
           left: 0.5em;
         }
      }

Expected behavior

It should not have a leading space:

      &.old {
         &::before {
           content: '-';
           position: absolute;
           left: 0.5em;
         }
      }

This was tested on Mac OS 10.8 Firefox 48.0.2

See also #14176 (closed)

Merge request !7133 (merged) should fix this.