Localize all the GroupBy*Date methods

Fixes #9745

hugolib/hugo_sites.go

@@ -73,6 +73,9 @@ type HugoSites struct {
 	// Render output formats for all sites.
 	renderFormats output.Formats
 
+	// The currently rendered Site.
+	currentSite *Site
+
 	*deps.Deps
 
 	gitInfo       *gitInfo

hugolib/hugo_sites_build.go

@@ -289,6 +289,7 @@ func (h *HugoSites) render(config *BuildCfg) error {
 
 	i := 0
 	for _, s := range h.Sites {
+		h.currentSite = s
 		for siteOutIdx, renderFormat := range s.renderFormats {
 			siteRenderContext.outIdx = siteOutIdx
 			siteRenderContext.sitesOutIdx = i

hugolib/site.go

@@ -738,6 +738,11 @@ func (s *SiteInfo) Sites() page.Sites {
 	return s.s.h.siteInfos()
 }
 
+// Current returns the currently rendered Site.
+func (s *SiteInfo) Current() page.Site {
+	return s.s.h.currentSite.Info
+}
+
 func (s *SiteInfo) String() string {
 	return fmt.Sprintf("Site(%q)", s.title)
 }