Add commit undo/redo

[skoonin]

Summary

Add commit undo/redo so a mistaken commit can be rolled back (and re-applied) with a simple action.

Desired behavior

• Undo the most recent commit (soft reset semantics, keeping changes in the working tree by default).
• Redo to re-apply an undone commit.
• Expose via an obvious control and/or the command palette.

Scope / open questions

• Define exactly which operations are undoable (start with commit; consider amend).
• Decide undo semantics (keep vs discard working-tree changes) and how far back the stack goes.

Follow-up PR targets develop.