Fix commits per tag counting.

A hard-to-spot bug where the total number of commits for a tag was always set
to the number of commits that one of the commiters did and not the cumulated
sum over all commiters.

[hoxu@users.sf.net: created a commit from diff and description sent by Thomas]

gitstats

 				parts = re.split('\s+', line, 2)
 				commits = int(parts[1])
 				author = parts[2]
-				self.tags[tag]['commits'] = commits
+				self.tags[tag]['commits'] += commits
 				self.tags[tag]['authors'][author] = commits
 
 		# Collect revision statistics