Print context ref if available

When specifying the expected fixed ref, we should print out the context ref in the log message if it's available.

Example of new message:
`Not migrating ref refs/heads/master, its fixed ref 831870a15a17cca4152ffde5c8a3ebc535c68ab0 did not match the expected fixed ref 711abb4fa89be8c77717e105e61eba5c20db4d81.`

BUG=380307584
PiperOrigin-RevId: 743653731
Change-Id: I3d4ca751596b8307ed27b24ca739f927c3a80d43

Author: chris-campos

java/com/google/copybara/Workflow.java

@@ -63,6 +63,7 @@
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
+import java.util.Optional;
 import java.util.Set;
 import java.util.concurrent.Callable;
 import java.util.function.Consumer;
@@ -344,7 +345,9 @@ private void validateExpectedFixedRef(O resolvedRef) throws EmptyChangeException
       throw new EmptyChangeException(
           String.format(
               "Not migrating ref %s, its fixed ref %s did not match the expected fixed ref %s.",
-              resolvedRef.asString(), resolvedRef.fixedReference(), expectedFixedRef));
+              Optional.ofNullable(resolvedRef.contextReference()).orElse(resolvedRef.asString()),
+              resolvedRef.fixedReference(),
+              expectedFixedRef));
     }
   }
 

javatests/com/google/copybara/WorkflowTest.java

@@ -400,7 +400,7 @@ public void expectedFixedRefDoesNotMatch() throws Exception {
     assertThat(e)
         .hasMessageThat()
         .contains(
-            "Not migrating ref 0, its fixed ref beaver did not match the expected fixed ref"
+            "Not migrating ref HEAD, its fixed ref beaver did not match the expected fixed ref"
                 + " capybara.");
   }