Expose additional trigger information from build cause

With this, it should be possible to use multiple different triggers and to
execute different steps based on the particular trigger. It also provides a way
to notify the user who triggered the build via GitHub comments.

The full comment is exposed so that trigger commands could be parameterized.
E.g. extracting the module name from `test module-x`, which matches a generic
`test.*` trigger.

README.md

@@ -92,6 +92,21 @@ pipeline {
 }
 ```
 
+### Detecting whether a build was started by the trigger in a script:
+```groovy
+def triggerCause = currentBuild.rawBuild.getCause(
+    org.jenkinsci.plugins.pipeline.github.trigger.IssueCommentCause
+)
+
+if (triggerCause) {
+    echo("Build was started by ${triggerCause.userLogin}, who wrote: " +
+         "\"${triggerCause.comment}\", which matches the " +
+         "\"${triggerCause.triggerPattern}\" trigger pattern.")
+} else {
+    echo('Build was not started by a trigger')
+}
+```
+
 # Global Variables
 
 ## `repository`

src/main/java/org/jenkinsci/plugins/pipeline/github/trigger/GitHubEventSubscriber.java

@@ -22,6 +22,7 @@
 import java.io.StringReader;
 import java.util.Collections;
 import java.util.HashSet;
+import java.util.Optional;
 import java.util.Set;
 
 /**
@@ -115,22 +116,24 @@ private void handleIssueComment(final GHSubscriberEvent event) {
         if (job == null) {
             LOG.debug("No job found matching key: {}", key);
         } else {
-            boolean matchingTriggers = job.getTriggersJobProperty()
+            Optional<IssueCommentTrigger> matchingTrigger = job.getTriggersJobProperty()
                     .getTriggers()
                     .stream()
                     .filter(t -> t instanceof IssueCommentTrigger)
                     .map(IssueCommentTrigger.class::cast)
                     .filter(t -> triggerMatches(t, issueCommentEvent.getComment(), job))
-                    .findAny()
-                    .map(t -> Boolean.TRUE)
-                    .orElse(Boolean.FALSE);
+                    .findAny();
 
-            if (matchingTriggers) {
+            if (matchingTrigger.isPresent()) {
                 String commentAuthor = issueCommentEvent.getComment().getUserName();
                 boolean authorized = isAuthorized(job, commentAuthor);
 
                 if (authorized) {
-                    job.scheduleBuild(new IssueCommentCause(issueCommentEvent.getComment().getUserName()));
+                    job.scheduleBuild(
+                            new IssueCommentCause(
+                                    issueCommentEvent.getComment().getUserName(),
+                                    issueCommentEvent.getComment().getBody(),
+                                    matchingTrigger.get().getCommentPattern()));
                     LOG.info("Job: {} triggered by IssueComment: {}",
                             job.getFullName(), issueCommentEvent.getComment());
                 } else {

src/main/java/org/jenkinsci/plugins/pipeline/github/trigger/IssueCommentCause.java

@@ -1,6 +1,7 @@
 package org.jenkinsci.plugins.pipeline.github.trigger;
 
 import hudson.model.Cause;
+import org.jenkinsci.plugins.scriptsecurity.sandbox.whitelists.Whitelisted;
 
 /**
  * Represents the user who authored the comment that triggered the build.
@@ -9,13 +10,32 @@
  */
 public class IssueCommentCause extends Cause {
     private final String userLogin;
+    private final String comment;
+    private final String triggerPattern;
 
-    public IssueCommentCause(final String userLogin) {
+    public IssueCommentCause(final String userLogin, final String comment, final String triggerPattern) {
         this.userLogin = userLogin;
+        this.comment = comment;
+        this.triggerPattern = triggerPattern;
+    }
+
+    @Whitelisted
+    public String getUserLogin() {
+        return userLogin;
+    }
+
+    @Whitelisted
+    public String getComment() {
+        return comment;
+    }
+
+    @Whitelisted
+    public String getTriggerPattern() {
+        return triggerPattern;
     }
 
     @Override
     public String getShortDescription() {
-        return String.format("Started by an IssueComment from user: %s", userLogin);
+        return String.format("%s commented: %s", userLogin, comment);
     }
 }