Updated merge request header

What does this MR do and why?

Updates the merge request header by moving the branch information from the widget into the header.

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Enable updated_mr_header

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• I have evaluated the MR acceptance checklist for this MR.

Related to #356703 (closed)

app/assets/javascripts/behaviors/shortcuts/shortcuts_issuable.js

@@ -156,7 +156,7 @@ export default class ShortcutsIssuable extends Shortcuts {
   static copyBranchName() {
     // There are two buttons - one that is shown when the sidebar
     // is expanded, and one that is shown when it's collapsed.
-    const allCopyBtns = Array.from(document.querySelectorAll('.js-sidebar-source-branch button'));
+    const allCopyBtns = Array.from(document.querySelectorAll('.js-source-branch-copy'));
 
     // Select whichever button is currently visible so that
     // the "Copied" tooltip is shown when a click is simulated.