Fix #323 merge_commit_sha field is missing in merge request event (#324)

* Added EventMergeRequest.mergeCommitSha field (#323)

Author: lesiak

src/main/java/org/gitlab4j/api/webhook/EventMergeRequest.java

@@ -18,6 +18,7 @@ public class EventMergeRequest {
     private String description;
     private Integer id;
     private Integer iid;
+    private String mergeCommitSha;
     private String mergeStatus;
     private Integer milestoneId;
     private Integer position;
@@ -97,6 +98,14 @@ public void setIid(Integer iid) {
         this.iid = iid;
     }
 
+    public String getMergeCommitSha() {
+        return mergeCommitSha;
+    }
+
+    public void setMergeCommitSha(String mergeCommitSha) {
+        this.mergeCommitSha = mergeCommitSha;
+    }
+
     public String getMergeStatus() {
         return this.mergeStatus;
     }

src/main/java/org/gitlab4j/api/webhook/WebHookListener.java

@@ -1,10 +1,6 @@
 
 package org.gitlab4j.api.webhook;
 
-import org.gitlab4j.api.webhook.IssueEvent;
-import org.gitlab4j.api.webhook.MergeRequestEvent;
-import org.gitlab4j.api.webhook.PushEvent;
-
 /**
  * This interface defines an event listener for the event fired when
  * a WebHook notification has been received from a GitLab server.

src/test/resources/org/gitlab4j/api/merge-request-event.json

@@ -40,6 +40,7 @@
     "merge_status": "unchecked",
     "target_project_id": 14,
     "iid": 1,
+    "merge_commit_sha": "5f72688f7af57df23ee7510cec59b909289964f1",
     "description": "",
     "source":{
       "name":"Awesome Project",