Suppress debug output when completing
Suppress debug output when completing

Any log output interferes with comletion.
We already refrain from installing our verbose
log handlers in this case, but we are still
vulnerable to GLib default message handling,
e.g. via the G_MESSAGES_DEBUG environment
variable. To avoid that, explicitly install
a message handler that suppresses all output.

Closes: https://github.com/flatpak/flatpak/issues/2860

Closes: #2862
Approved by: alexlarsson

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