Fix running gitstats with a non-tty stdin.

The following command used to fail because git shortlog has some witty magic to
decide the behaviour based on whether stdin is a terminal:

./gitstats /repo /dst < /dev/null

effectively causing a lot of shell scripts calling gitstats to fail with
"Division by zero" in author statistics.

Thanks-to: Bertrand Jacquin <beber@meleeweb.net>
Thanks-to: Sylvain Rabot <sylvain@abstraction.fr>

gitstats

 	exectime_external += (end - start)
 	return output.rstrip('\n')
 
-def getcommitrange(defaultrange = '', end_only = False):
+def getcommitrange(defaultrange = 'HEAD', end_only = False):
 	if len(conf['commit_end']) > 0:
 		if end_only or len(conf['commit_begin']) == 0:
 			return conf['commit_end']