[github_pr_destination] Don't require context reference if pr_branch is set

This will allow copybara to succeed for the following origin:

```
git.origin(
    url = "https://github.com/google/copybara.git",
    ref = "b0f6c6bbb5828c95b2b1409b4e491865d969f679",
)
```

Author: Yannic

java/com/google/copybara/git/GitHubPrDestination.java

@@ -337,14 +337,18 @@ private String getPullRequestBranchName(
       return gitHubDestinationOptions.destinationPrBranch;
     }
     String contextReference = changeRevision.contextReference();
-    // We could do more magic here with the change identity. But this is already complex so we
-    // require  a group identity either provided by the origin or the workflow (Will be implemented
-    // later.
-    checkCondition(contextReference != null,
-        "git.github_pr_destination is incompatible with the current origin. Origin has to be"
-            + " able to provide the contextReference or use '%s' flag",
-        GitHubDestinationOptions.GITHUB_DESTINATION_PR_BRANCH);
-    String branchNameFromUser = getCustomBranchName(contextReference);
+
+    String branchNameFromUser = prBranch;
+    if (prBranch == null) {
+      // We could do more magic here with the change identity. But this is already complex so we
+      // require a group identity either provided by the origin or the workflow (Will be implemented
+      // later.
+      checkCondition(contextReference != null,
+          "git.github_pr_destination is incompatible with the current origin. Origin has to be"
+              + " able to provide the contextReference or use '%s' flag",
+          GitHubDestinationOptions.GITHUB_DESTINATION_PR_BRANCH);
+      branchNameFromUser = getCustomBranchName(contextReference);
+    }
     String branchName =
         branchNameFromUser != null
             ? branchNameFromUser