Include HEAD upon git.destination NOOP

Add the current head revision to the output when encountering a noop.
This should make it easier to understand what the resulting state is.

PiperOrigin-RevId: 299945189
Change-Id: Idb9f5f824f04c325c720e20ad61e80377c6ca963

Author: hsudhof

java/com/google/copybara/git/GitRepository.java

@@ -788,8 +788,10 @@ public void commit(@Nullable String author, boolean amend, @Nullable ZonedDateTi
       String message)
       throws RepoException, ValidationException {
     if (isEmptyStaging() && !amend) {
-      throw new EmptyChangeException("Migration of the revision resulted in an empty change. "
-          + "Is the change already migrated?");
+      throw new EmptyChangeException(
+          String.format(
+              "Migration of the revision resulted in an empty change from baseline '%s'.\n"
+                  + "Is the change already migrated?", parseRef("HEAD")));
     }
 
     ImmutableList.Builder<String> params = ImmutableList.<String>builder().add("commit");

javatests/com/google/copybara/WorkflowTest.java

@@ -1573,7 +1573,7 @@ public void testDryRunWithLocalGitPath() throws Exception {
         + ")\n";
 
     addGitFile(originPath, origin, "foo.txt", "not important");
-    commit(origin, "baseline");
+    commit(origin, "baseline\n\nOrigin-Label: 1234567");
 
     options.setWorkdirToRealTempDir();
     // Pass custom HOME directory so that we run an hermetic test and we
@@ -1605,6 +1605,9 @@ public void testDryRunWithLocalGitPath() throws Exception {
     assertThat(e)
         .hasMessageThat()
         .contains("Migration of the revision resulted in an empty change");
+    assertThat(e)
+        .hasMessageThat()
+        .contains(destination.parseRef("HEAD"));
   }
 
   private void checkChangeRequest_sot_ahead_sot()