Remove backticks from author names passed to gnuplot.

Without this, author names containing `touch /tmp/vulnerable` would cause said
file to appear after generating statistics for the given repository.

This is not an optimal solution. Instead of blacklisting characters we should
either whitelist some, or find a safe escape mechanism for gnuplot.

gitstats

 		plots = []
 		for a in self.authors_to_plot:
 			i = i + 1
-			plots.append("""'lines_of_code_by_author.dat' using 1:%d title "%s" w lines""" % (i, a.replace("\"", "\\\"")))
+			author = a.replace("\"", "\\\"").replace("`", "")
+			plots.append("""'lines_of_code_by_author.dat' using 1:%d title "%s" w lines""" % (i, author))
 		f.write(", ".join(plots))
 		f.write('\n')
 
 		plots = []
 		for a in self.authors_to_plot:
 			i = i + 1
-			plots.append("""'commits_by_author.dat' using 1:%d title "%s" w lines""" % (i, a.replace("\"", "\\\"")))
+			author = a.replace("\"", "\\\"").replace("`", "")
+			plots.append("""'commits_by_author.dat' using 1:%d title "%s" w lines""" % (i, author))
 		f.write(", ".join(plots))
 		f.write('\n')