Hide Git output when running Drush commands

This is a follow-up to #1835444: Support the new dev info strings.

When processing modules with dev releases, the following output can be seen when running drush commands:

colan@snake[Mon 29 17:31]% drush hacked-list-projects --force-rebuild
Rebuilding Hacked! report
Finished processing: Admin Toolbar [ok]
Finished processing: Drupal core [ok]
Finished processing: Configuration Update Manager [ok]
Cloning into '8.x-3.0-alpha5+23-dev'...
Note: checking out '8ff7541969abe7bbc689331948629eb1b26c01cd'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b <new-branch-name>

HEAD is now at 8ff7541... Issue #2637988 by felribeiro, ashhishhh: Features should be marked as uninstalled not disabled
Finished processing: Features [ok]
[...]

On the exec() calls, We should use the --quiet option on both git clone and git checkout to silence messages such as the above.