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.
d6a8217e94