completion: avoid potential hangs in dnf_commands_helper()

The completion helper can hang indefinitely waiting for user input
(which the user cannot see).  The --assumeno option ensures any prompts
are answered negatively.

Using the --nogpgcheck option avoids the helper downloading gpgkeys for
repos which have repo_gpgcheck=1 set.  This can speed up the completion
quite a bit.

Lastly, redirect input from /dev/null to ensure the completion_helper
command knows it has no controlling tty on which to wait for input.

Closes: #894
Approved by: m-blaha

etc/bash_completion.d/dnf

@@ -120,7 +120,7 @@ _dnf_set_python_exec()
 
 _dnf_commands_helper()
 {
-    echo "$( ${__dnf_python_exec} -c "import sys; from dnf.cli import completion_helper as ch; ch.main(sys.argv[1:])" "$@" -d 0 -q -C 2>/dev/null )"
+    echo "$( ${__dnf_python_exec} -c "import sys; from dnf.cli import completion_helper as ch; ch.main(sys.argv[1:])" "$@" -d 0 -q -C --assumeno --nogpgcheck 2>/dev/null </dev/null )"
 }
 
 _dnf_is_path()