Fixed incorrect creation of relative submodule URL (#14)

According to git submodule documentation base path for relative submodule URL
is parent repository's root - not submodule "mount point".

So it's necessary to know "path" of parent repository to create correct submodule
URL.

submodule.go

@@ -29,7 +29,7 @@ func NewSubModuleFile(c *Commit, refUrl, refId string) *SubModuleFile {
 
 // FIXME: remove import of setting
 // RefUrl guesses and returns reference URL.
-func (sf *SubModuleFile) RefUrl(urlPrefix string) string {
+func (sf *SubModuleFile) RefUrl(urlPrefix string, parentPath string) string {
 	if sf.refUrl == "" {
 		return ""
 	}
@@ -46,6 +46,14 @@ func (sf *SubModuleFile) RefUrl(urlPrefix string) string {
 		return url
 	}
 
+	// relative url prefix check (according to git submodule documentation)
+	if strings.HasPrefix(url, "./") || strings.HasPrefix(url, "../") {
+		// ...construct and return correct submodule url here...
+		idx := strings.LastIndex(parentPath, "/src/")
+		rel_url := strings.TrimSuffix(urlPrefix, "/") + parentPath[:idx] + "/" + url
+		return rel_url
+	}
+
 	// sysuser@xxx:user/repo
 	i := strings.Index(url, "@")
 	j := strings.LastIndex(url, ":")