feat: add button to quickly clear merge message (#21548)

This PR adds a button to allow quickly clearing the merge message of a
PR. The button will remove everything but the git trailers.

I found myself often pruning the commit message before merging,
especially for PRs generated by renovate - renovate puts a very long and
detailed comment with the full changelog in each PR it opens. This
clutters the commit message. However, I want to explicitly preserve the
git commit trailers. Doing this manually works, but having a button is a
lot easier.

Screenshot:

![image](https://user-images.githubusercontent.com/13721712/197337525-d456d0f8-1f7c-43a9-815d-ca93b1e7a90a.png)

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: delvh <dev.lh@web.de>

web_src/js/components/PullRequestMergeForm.vue

@@ -25,6 +25,14 @@
           </div>
           <div class="field">
             <textarea name="merge_message_field" rows="5" :placeholder="mergeForm.mergeMessageFieldPlaceHolder" v-model="mergeMessageFieldValue"/>
+            <template v-if="mergeMessageFieldValue !== mergeForm.defaultMergeMessage">
+              <button @click.prevent="clearMergeMessage" class="ui tertiary button">
+                {{ mergeForm.textClearMergeMessage }}
+              </button>
+              <div class="ui label"><!-- TODO: Convert to tooltip once we can use tooltips in Vue templates -->
+                {{ mergeForm.textClearMergeMessageHint }}
+              </div>
+            </template>
           </div>
         </template>
 
@@ -174,6 +182,9 @@ export default {
       this.mergeStyle = name;
       this.autoMergeWhenSucceed = autoMerge;
     },
+    clearMergeMessage() {
+      this.mergeMessageFieldValue = this.mergeForm.defaultMergeMessage;
+    },
   },
 };
 </script>