Memoize absolute_url and relative_url filters

These two filters are quite slow, as Addressable::URI.parse is quite
slow. Fortunately, they are pure functions and so can be easily
memoized. This one change shaves 8s off the CockroachDB docs build
(github.com/cockroachdb/docs).

lib/jekyll/filters/url_filters.rb

@@ -10,16 +10,20 @@ module URLFilters
       # Returns the absolute URL as a String.
       def absolute_url(input)
         return if input.nil?
-
-        input = input.url if input.respond_to?(:url)
-        return input if Addressable::URI.parse(input.to_s).absolute?
-
-        site = @context.registers[:site]
-        return relative_url(input) if site.config["url"].nil?
-
-        Addressable::URI.parse(
-          site.config["url"].to_s + relative_url(input)
-        ).normalize.to_s
+        cache = (@context.registers[:cached_absolute_urls] = {})
+        cache[input] ||= begin
+          input = input.url if input.respond_to?(:url)
+          site = @context.registers[:site]
+          if Addressable::URI.parse(input.to_s).absolute?
+            input
+          elsif site.config["url"].nil?
+            relative_url(input)
+          else
+            Addressable::URI.parse(
+              site.config["url"].to_s + relative_url(input)
+            ).normalize.to_s
+          end
+        end
       end
 
       # Produces a URL relative to the domain root based on site.baseurl
@@ -30,14 +34,18 @@ def absolute_url(input)
       # Returns a URL relative to the domain root as a String.
       def relative_url(input)
         return if input.nil?
-
-        input = input.url if input.respond_to?(:url)
-        return input if Addressable::URI.parse(input.to_s).absolute?
-
-        parts = [sanitized_baseurl, input]
-        Addressable::URI.parse(
-          parts.compact.map { |part| ensure_leading_slash(part.to_s) }.join
-        ).normalize.to_s
+        cache = (@context.registers[:cached_relative_urls] ||= {})
+        cache[input] ||= begin
+          input = input.url if input.respond_to?(:url)
+          if Addressable::URI.parse(input.to_s).absolute?
+            input
+          else
+            parts = [sanitized_baseurl, input]
+            Addressable::URI.parse(
+              parts.compact.map { |part| ensure_leading_slash(part.to_s) }.join
+            ).normalize.to_s
+          end
+        end
       end
 
       # Strips trailing `/index.html` from URLs to create pretty permalinks