feat(git): add diff, branch, checkout, push, pull to Git sidebar panel

Extends GitOperations.js with diff/branches/checkoutBranch/push/pull,
wires the 5 new IPC handlers in main.js (reusing the existing dir
resolution), whitelists the new channels in preload.js, and fixes the
Git sidebar panel's previously dead _gitDiff callback by wiring up a
diff view, branch list/create/checkout UI, and push/pull buttons.
Resolves Task 5, which deferred this work to this task.

Amit Haridas

src/preload.js

@@ -107,6 +107,10 @@ const ALLOWED_SEND_CHANNELS = [
   'git-stage',
   'git-commit',
   'git-log',
+  'git-branches',
+  'git-checkout',
+  'git-push',
+  'git-pull',
 
   // Snippets
   'get-snippets',