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