`composer update -h` triggers prefetch

[troggybrains]

As stated in the title, issuing a composer update -h on commandline, prestissimo will trigger a prefetch. On projects with a large amount of dependencies (e.g. Magento 2), completion can take a while until finished and until we get what we actually want to see, the help message of the subcommand update. Usually you wouldn't issue a composer update -h but a very neat bash-autocompletion script for composer does it every time you type composer update <tab> in order to present options for the subcommand.

Reproduction:

1. Install prestissimo as described
2. Change directory to a project with lots of dependencies
3. Issue composer update -h => Watch and wait until presitissimo has prefetched all dependencies