Fix custom name in branch creation for issue in Firefox

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/41563

`event.srcElement` is non-standard,
https://developer.mozilla.org/en-US/docs/Web/API/Event/srcElement

app/assets/javascripts/create_merge_request_dropdown.js

@@ -276,13 +276,13 @@ export default class CreateMergeRequestDropdown {
     let target;
     let value;
 
-    if (event.srcElement === this.branchInput) {
+    if (event.target === this.branchInput) {
       target = 'branch';
       value = this.branchInput.value;
-    } else if (event.srcElement === this.refInput) {
+    } else if (event.target === this.refInput) {
       target = 'ref';
-      value = event.srcElement.value.slice(0, event.srcElement.selectionStart) +
-        event.srcElement.value.slice(event.srcElement.selectionEnd);
+      value = event.target.value.slice(0, event.target.selectionStart) +
+        event.target.value.slice(event.target.selectionEnd);
     } else {
       return false;
     }