Simplify `branch_name` in bin/changelog

The --short option has existed since at least 1.8.1: https://git-scm.com/docs/git-symbolic-ref/1.8.1

bin/changelog

@@ -158,7 +158,7 @@ class ChangelogEntry
   end
   def branch_name
-    @branch_name ||= %x{git symbolic-ref HEAD}.strip.sub(%r{\Arefs/heads/}, '')
+    @branch_name ||= %x{git symbolic-ref --short HEAD}.strip
   end
 end