[JENKINS-61290] hard to see why a branch was not inspected - log them

BranchDiscoveryTrait.java : report into scan log if a branch is ignored
(because it is or is-not a source of PR, based on user choice of settings)
similarly as proposed and refined for github-branch-source-plugin in
jenkinsci/github-branch-source-plugin#284

Author: jimklimov

src/main/java/com/cloudbees/jenkins/plugins/bitbucket/BranchDiscoveryTrait.java

@@ -245,6 +245,12 @@ public boolean isExcluded(@NonNull SCMSourceRequest request, @NonNull SCMHead he
                     BitbucketRepository source = pullRequest.getSource().getRepository();
                     if (fullName.equalsIgnoreCase(source.getFullName())
                             && pullRequest.getSource().getBranch().getName().equals(head.getName())) {
+                        // End the format with newline to avoid logging this
+                        // result blocked together with a later indexed branch
+                        request.listener().getLogger().format(
+                                "Ignoring %s because current strategy excludes branches "
+                                + "that ARE also filed as a pull request%n"
+                                , head.toString());
                         return true;
                     }
                 }
@@ -272,6 +278,10 @@ public boolean isExcluded(@NonNull SCMSourceRequest request, @NonNull SCMHead he
                         return false;
                     }
                 }
+                request.listener().getLogger().format(
+                        "Ignoring %s because current strategy excludes branches "
+                        + "that ARE NOT also filed as a pull request%n"
+                        , head.toString());
                 return true;
             }
             return false;