Fixing commit_begin conf option

If you tried to pass only -c commit_begin=something this would
always return HEAD in any case since commit_end was set to ''.

Defaulting commit_end to HEAD makes getcommitrange() function work as it
should.

Signed-off-by: Heikki Hokkanen <hoxu@users.sf.net>

gitstats

 	'max_authors': 20,
 	'authors_top': 5,
 	'commit_begin': '',
-	'commit_end': '',
+	'commit_end': 'HEAD',
 	'linear_linestats': 1,
 	'project_name': '',
 }