Explain when a given reference is not valid

bin/changelog

@@ -1,5 +1,11 @@
 #!/usr/bin/env ruby
 
+def silent_run(cmd)
+  pid = spawn(cmd, out: "/dev/null", err: "/dev/null")
+  Process.wait(pid)
+  $?.success?
+end
+
 if ARGV.count != 1
   script = $0
   puts "Usage:"
@@ -12,6 +18,11 @@ end
 
 gitref = ARGV.first
 
+unless silent_run("git cat-file -t #{gitref}")
+  puts "The given git reference `#{gitref}` does not exist in this repository."
+  exit 1
+end
+
 files = `git diff --name-only #{gitref} | grep html.erb`
 
 if files.empty?