Isolate hack for skipping internal separator

stage_wildcards has no need for INTERNAL_SEPARATOR. Remove it already
at this stage, to avoid the need to have the workaround in the generic
decoding routine.

Unfortunately we still can't add assert!(!fish_reserved_codepoint(c));
to wcs2bytes_callback, since builtin printf still passes arbitrary
characters. We should fix that later.

Test that printf now behaves correctly for INTERNAL_SEPARATOR.
Also add a regression test for 0c9b73e317.

tests/checks/basic.fish

@@ -630,6 +630,9 @@ $fish -c 'echo \ufdd2"fart"'
 echo (printf '\ufdd2foo') | string escape
 # CHECK: \Xef\Xb7\X92foo
 
+echo (printf '\ufdd8foo') | string escape
+# CHECK: \Xef\Xb7\X98foo
+
 printf '%s\n' "#!/bin/sh" 'echo $0' > $tmpdir/argv0.sh
 chmod +x $tmpdir/argv0.sh
 cd $tmpdir

tests/checks/expansion.fish

@@ -350,3 +350,17 @@ echo {asdf,~}
 # CHECK: asdf /{{.*}}
 echo {~}
 # CHECK: {~}
+
+function compare
+    test $argv[1] = $argv[2]
+    or begin
+        echo unexpected expansion result:
+        echo expected: $argv[1]
+        echo actual: $argv[2]
+    end
+end
+if string match -rq -- '^[\w.-]+$' $USER
+    set -l user_home "$(eval "echo ~$USER")"
+    compare $user_home "$(echo ~$USER)"
+    compare $user_home "$(echo ~(printf %s $USER))"
+end