Fix for KeyError being generated if a month has commits but for some reason no lines changed
Example of exception
[1.17747] >> gnuplot --version
Traceback (most recent call last):
File "./gitstats", line 1373, in <module>
g.run(sys.argv[1:]
File "./gitstats", line 1365, in run
report.create(data, outputpath)
File "./gitstats", line 841, in create
f.write('<tr><td>%s</td><td>%d</td><td>%d</td><td>%d</td></tr>' % (yymm, data.commits_by_month[yymm], data.lines_added_by_month[yymm],
data.lines_removed_by_month[yymm]))
KeyError: '2010-08'
Signed-off-by: Heikki Hokkanen <hoxu@users.sf.net>