fix: bitbucket service pull request build status from different source

The commit 7ac886d switched to the newer repository commit build
status api. This introduced a regression in the case the PR source
repository is not the same as the target, for example a person fork.
In this case the bitbucket api created from the PR head will point to
the fork repo and may result in a 401 error if the credentials don't
have the necessary permissions.

```
ERROR: Could not send notifications
com.cloudbees.jenkins.plugins.bitbucket.api.BitbucketRequestException: HTTP request error.
Status: HTTP/1.1 401
Response: {"errors":[{"context":null,"message":"You are not permitted to access this resource","exceptionName":"com.atlassian.bitbucket.AuthorisationException"}]}
```

This change will force the bitbucket api instance to be the target repo
which is the appropriate target for the notification. The old api was
commit hash centric, not project centric, so the api client owner/repo
didn't matter.

This change also reverts some changes introduced by jenkinsci#954 to the
refName parameter. While local testing has shown that by changing
the bitbucket client to point to the target repo, setting the refName
to null works in all cases, this change has been restricted to only
cases when the source repo is the same as the target.

No changes were made to the Bitbucket Cloud code path.

Author: cronik

src/main/java/com/cloudbees/jenkins/plugins/bitbucket/impl/notifier/BitbucketBuildStatusNotifications.java

@@ -229,19 +229,31 @@ private static void sendNotifications(BitbucketSCMSource source, Run<?, ?> build
              * Poor documentation for bitbucket cloud at:
              * https://community.atlassian.com/t5/Bitbucket-questions/Re-Builds-not-appearing-in-pull-requests/qaq-p/1805991/comment-id/65864#M65864
              * that means refName null or valued with only head.getBranchName()
-             *
-             * For Bitbucket Server, refName should be "refs/heads/" + the name
-             * of the source branch of the pull request, and the build status
-             * should be posted to the repository that contains that branch.
-             * If refName is null, then Bitbucket Server does not show the
-             * build status in the list of pull requests, but still shows it
-             * on the web page of the individual pull request.
              */
-            bitbucket = source.buildBitbucketClient(head);
-            if (BitbucketApiUtils.isCloud(bitbucket)) {
-                refName = null;
+            refName = null;
+            if (BitbucketApiUtils.isCloud(source.getServerUrl())) {
+                bitbucket = source.buildBitbucketClient(head);
             } else {
-                refName = "refs/heads/" + head.getBranchName();
+                // For  Bitbucket Server, notify the target of the PR. Head may point to a forked source repo that the
+                // credentials don't have access to resulting in a 401 error.
+                bitbucket = source.buildBitbucketClient();
+                // For Bitbucket Server, refName should be "refs/heads/" + the name
+                // of the source branch of the pull request, and the build status
+                // should be posted to the repository that contains that branch.
+                // If refName is null, then Bitbucket Server does not show the
+                // build status in the list of pull requests, but still shows it
+                // on the web page of the individual pull request.
+                //
+                // While the above behavior has been reported by some users, it only
+                // applies where the target branch exists on the target repo.
+                // Other tests have shown that refName null works no matter
+                // the source of the pull request when updating the build status
+                // on the pull request target, but will continue to set refName
+                // to account for the reported behavior.
+                // https://developer.atlassian.com/server/bitbucket/rest/v905/api-group-builds-and-deployments/#api-api-latest-projects-projectkey-repos-repositoryslug-commits-commitid-builds-post
+                if (StringUtils.equals(head.getRepoOwner(), source.getRepoOwner()) && StringUtils.equals(head.getRepository(), source.getRepository())) {
+                    refName = "refs/heads/" + head.getBranchName();
+                }
             }
         } else {
             listener.getLogger().println("[Bitbucket] Notifying commit build result");