✨ Highlight "Only in" lines

When viewing a recursive directory diff, highlight lines showing that a
file is contained in only one of of the sides.

Closes #5

Signed-off-by: Mike Fiedler <[email protected]>

Author: miketheman

styles/diff.less

@@ -12,5 +12,11 @@ atom-text-editor::shadow {
     &.deleted {
       color: @syntax-color-removed;
     }
+
+    &.only-in {
+      // we don't have any methods in diff, so we can safely use this color
+      color: @syntax-color-method;
+      font-weight: bold;
+    }
   }
 }