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

@@ -0,0 +1,30 @@
+# vim: set filetype=expect:
+
+spawn $fish
+
+expect_prompt
+
+send_line "bind '~' 'handle_tilde'"
+expect_prompt
+
+# printing the current buffer should not remove quoting
+send_line "function handle_tilde; echo; echo '@GUARD:1@'; commandline -b; echo '@/GUARD:1@'; commandline -b ''; end"
+expect_prompt
+send_line {echo \en one "two three" four'five six'{7} 'eight~}
+expect_prompt -re {\r\n@GUARD:1@\r\n(.*)\r\n@/GUARD:1@\r\n} {
+    puts "a:"
+    puts $expect_out(1,string)
+} unmatched {
+    abort "Couldn't find guard 1"
+}
+
+# printing the buffer with -o should remove quoting
+send_line "function handle_tilde; echo; echo '@GUARD:2@'; commandline -bo; echo '@/GUARD:2@'; commandline -b ''; end"
+expect_prompt
+send_line {echo one "two three" four'five six'{7} 'eight~}
+expect_prompt -re {\r\n@GUARD:2@\r\n(.*)\r\n@/GUARD:2@\r\n} {
+    puts "b:"
+    puts [string map {\r {}} $expect_out(1,string)]
+} unmatched {
+    abort "Couldn't find guard 2"
+}