Fix zsh completion (#1108)

4ec533b introduced a bug which causes the 'hidden' cli parameter
`__complete` to be ignored.

Add a check for this parameter, so that it can pass to `cobra`

Signed-off-by: Tom Whitwell <tom@whi.tw>

acceptance_tests/completion.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+
+testCompletionRuns() {
+    result=$(./yq __complete "" 2>&1)
+    assertEquals 0 $?
+    assertContains "$result" "Completion ended with directive:"
+}
+
+source ./scripts/shunit2