Fix diff command content (#2036)

Fix diff command getContent which returned always empty string because the results were ignored.

gradle/changelog/diff_command_content.yaml

@@ -0,0 +1,2 @@
+- type: fixed
+  description: Diff command content ([#2036](https://github.com/scm-manager/scm-manager/pull/2036))

scm-core/src/main/java/sonia/scm/repository/api/DiffCommandBuilder.java

@@ -110,7 +110,7 @@ public final class DiffCommandBuilder extends AbstractDiffCommandBuilder<DiffCom
    */
   public String getContent() throws IOException {
     try (ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
-      getDiffResult();
+      getDiffResult().accept(baos);
       return baos.toString();
     }
   }