Fix IDE not showing diff when opening commit tab

Part of this is caused by the removal of "changed"
and "staged" split.

**How come?**

When there was this split, it was acceptable to
see "what's changed compared to staged (or HEAD)".
Now that there is only "staged", when we open we
were unfortunately comparing the "staged" content
against intself...

app/assets/javascripts/ide/components/repo_commit_section.vue

@@ -45,7 +45,7 @@ export default {
     if (this.lastOpenedFile && this.lastOpenedFile.type !== 'tree') {
       this.openPendingTab({
         file: this.lastOpenedFile,
-        keyPrefix: this.lastOpenedFile.changed ? stageKeys.unstaged : stageKeys.staged,
+        keyPrefix: this.lastOpenedFile.staged ? stageKeys.staged : stageKeys.unstaged,
       })
         .then(changeViewer => {
           if (changeViewer) {

changelogs/unreleased/199195-ide-fix-diff-highlighting.yml

+---
+title: Fix Web IDE not showing diff when opening commit tab
+merge_request: 29439
+author:
+type: fixed