Add confirmation dialog to checkoutHead

There have been a few reports of files mysteriously going back to the
last version in the tree. My theory is that it is because of people
fat-fingering the Undo command because the default keyboard mapping is
too similar. This will at least prevent most accidental data loss.
Add config option for checkoutHead confirmation

Per the discussion with @kevinsawicki in #3168, I have added a config
option `editor.confirmCheckoutHead` that defaults to `true`. When the
`editor:checkout-head-revision` command is executed, a confirmation
dialog now shows that states which file is to be reverted and asks the
user to "Revert" or "Cancel". If the config option is set to `false`,
the old behavior, simply reverting without prompting, is used.

I also added tests to ensure that the confirmation dialog is displayed
or not in the right configurations.
Add a step to revert unsaved changes

I took the key part from PR #3183 and added it to the post-confirmation
flow.
Merge pull request #3169 from lee-dohm/ld-checkout-head

Add confirmation dialog to checkoutHead