Rewrite __fish_urlencode to not encode valid characters

Much better to only encode the characters that are not URL-safe. This
also doesn't involve any forking, and it even handles newlines and NULs
in the input.

share/functions/__fish_urlencode.fish

@@ -1,11 +1,22 @@
 function __fish_urlencode --description "URL-encode stdin"
-	set -l output
+	set -l join ''
 	set -l chars
 	# Set locale to C and IFS to "" in order to split a line into bytes.
-	while begin; set -lx LC_ALL C; set -lx IFS ''; read --array chars; end
+	while begin; set -lx LC_ALL C; set -lx IFS ''; read -az chars; end
-		if count $chars > /dev/null
+		printf '%s' $join
-			set output $output (printf '%%%02x' "'"$chars)
+		# chomp off a trailing newline
+		if test "$chars[-1]" = \n
+			set -e chars[-1]
+			set join '%0A%00'
+		else
+			set join '%00'
+		end
+		for c in $chars
+			if string match -q -r '[/._~A-Za-z0-9-]' $c
+				printf '%s' $c
+			else
+				printf '%%%02X' "'$c"
+			end
 		end
 	end
-	echo -s $output | sed -e 's/%2[fF]/\//g'
 end