fix: fetch before merging in Update from Default Branch

Without a fetch, the operation compared against the stale local ref for
the default branch, causing it to incorrectly report "already up to
date" when remote had unpulled commits.

Fixes #110

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: guplem

app/src/ui/app.tsx

@@ -640,7 +640,7 @@ export class App extends React.Component<IAppProps, IAppState> {
     // Disable autoupdates so that the app doesn't revert to the desktop/desktop upstream whenever there is an update.
   }
 
-  private updateBranchWithContributionTargetBranch() {
+  private async updateBranchWithContributionTargetBranch() {
     const { selectedState } = this.state
     if (
       selectedState == null ||
@@ -659,6 +659,8 @@ export class App extends React.Component<IAppProps, IAppState> {
       return
     }
 
+    await this.props.dispatcher.fetch(repository, FetchType.UserInitiatedTask)
+
     this.props.dispatcher.initializeMergeOperation(
       repository,
       false,