Support subdir in baseurl.

Mainly this was a change to helpers.MakePermalink, but to get the local server to run correctly,
we needed to redirect the path of the request from /foo to /.  In addition, I added tests for the
server's code for fixing up the base url with different config file & CLI options.

helpers/url.go

@@ -14,8 +14,10 @@
 package helpers
 
 import (
+	"fmt"
 	"net/url"
 	"path"
+	"strings"
 
 	"github.com/PuerkitoBio/purell"
 )
@@ -57,11 +59,23 @@ func MakePermalink(host, plink string) *url.URL {
 		panic(err)
 	}
 
-	path, err := url.Parse(plink)
+	p, err := url.Parse(plink)
 	if err != nil {
 		panic(err)
 	}
-	return base.ResolveReference(path)
+
+	if p.Host != "" {
+		panic(fmt.Errorf("Can't make permalink from absolute link %q", plink))
+	}
+
+	base.Path = path.Join(base.Path, p.Path)
+
+	// path.Join will strip off the last /, so put it back if it was there.
+	if strings.HasSuffix(p.Path, "/") && !strings.HasSuffix(base.Path, "/") {
+		base.Path = base.Path + "/"
+	}
+
+	return base
 }
 
 func UrlPrep(ugly bool, in string) string {