Stop unescaping strings with commandline -b

The fix for #2075 inadvertently started unescaping the strings emitted
from `commandline -b`. Only strings emitted with the `-o` flag are
supposed to be unescaped.

Fixes #2210.

tests/commandline.expect.out

@@ -0,0 +1,8 @@
+a:
+echo \en one "two three" four'five six'{7} 'eight
+b:
+echo
+one
+two three
+fourfive six{7}
+eight