fish_config theme dump: speed up

For better or worse, "set -L" prints all of $history, which makes
"fish_config theme show" very slow.  Fix this by only printing the
relevant variables.  While at, make the escaping function use the
shared subset of fish and POSIX shell quoting syntax, to allow a
following commit to use shlex.split().

share/functions/__fish_posix_quote.fish

@@ -0,0 +1,11 @@
+# localization: skip(private)
+function __fish_posix_quote
+    for arg in $argv
+        if set -l bareword (string match -r -- '^[-=\w]+$' $arg)
+            printf "%s " $bareword
+            continue
+        end
+        printf "'%s' " (printf %s $arg | string replace -a -- "'" "'\\''" |
+            string collect --no-trim-newlines)
+    end
+end

share/functions/fish_config.fish

@@ -190,8 +190,11 @@ function fish_config --description "Launch fish's web based configuration"
                         echo "Too many arguments" >&2
                         return 1
                     end
-                    # Write the current theme in .theme format, to stdout.
-                    set -L | string match -r (__fish_theme_variable_filter)
+                    # Write some of the current theme in .theme format, to stdout.
+                    for varname in (__fish_theme_variables)
+                        __fish_posix_quote $varname $$varname
+                        echo
+                    end
                 case '*'
                     echo "No such command: $cmd" >&2
                     return 1