make changes_github: use details tag to make changelog collapsible.

Makefile

@@ -55,7 +55,7 @@ fix_spacing:
 	for f in $(SOURCEFILES); do printf '%s\n' "`cat $$f`" | sed -e 's/  *$$//' > $$f.tmp; mv $$f.tmp $$f; done
 
 changes_github:
-	pandoc --filter tools/extract-changes.hs changelog.md -t gfm --wrap=none | sed -e 's/\\#/#/g' | pbcopy
+	pandoc --filter tools/extract-changes.hs changelog.md -t gfm --wrap=none --template tools/changes_template.html | sed -e 's/\\#/#/g' | pbcopy
 
 dist: man/pandoc.1
 	cabal sdist

tools/changes_template.html

@@ -0,0 +1,6 @@
+<details>
+  <summary>Click to expand changelog</summary>
+
+$body$
+
+</details>