Bump copyright year.
Fix performance issue for huge repositories

Problem: gitstats will read every commit and every file in repository in one
thread during initial statistics generation (i.e. no cache available). It may
take much time in case of huge repositories (100 000+ files) Solution: Execute
all read commands in 24 threads instead of one

Signed-off-by: Heikki Hokkanen <hoxu@users.sf.net>
Make number of processes configurable.

Default to 8.