Add fetch actions next to both branch selectors

[iambinlin]

Summary

Add a fetch action next to each of the two branch selectors in the branch comparison UI.

Motivation

Users should be able to refresh remote refs directly where they choose the source and target branches, without leaving the comparison flow.

Expected behavior

• Show a Fetch button or icon next to both branch selectors.
• Trigger a fetch for the relevant repository/remote when the user clicks either action.
• Preserve the current branch selections after fetching whenever possible.
• Show a loading or disabled state while fetching.
• Surface a clear error message if the fetch fails.

Notes

This should make it easier to compare against newly pushed branches or recently updated remote branches.