Add safeguard for out of sync loadBackup

[maxyu1115]

Currently loading backups will directly load whatever is on the remote backup branch. However, this is not safe to do when the remote backup branch is ahead of your actual branch.

For example, say I created a backup branch gbs-backup-example for my branch example. If my local example branch is at commit X. and my remote/gbs-backup-example is at commit X+2, we shouldn't directly bring in the work in progress from the remote.

Instead, after we detect such a scenario, we should create a stern warning for the user with a popup window, ask if they are certain, and only proceed when they confirm.

[maxyu1115]

This safeguard would need to be written here:

git-backup-sync/src/extension.ts

Line 310 in 5566ba2

// undoes the last commit

This is right after we pull in the remote backup branch contents, and before we bring over the change to our working branch.