Fix the working directory for the second git command (execute it in git repository, like the first command)

The script was throwing an error when building chart "authors" with
Linux git repository: some lines with author have a count, but no name.

The script was throwing an error when building chart "authors" with
Linux git repository (some names are not UTF-8 valid). The decoding
errors are now ignored.

New chart "commits_day" shows commits by day (by default the 20 last
days with at least one commit).

Two charts have been renamed:
- commits_hour -> commits_hour_day
- commits_day -> commits_day_week

New command line option -s/--sort-max: keep max entries in chart and
sort them by value.

Option -m/--max has been renamed to -d/--max-diff and is now used as
well for the new chart "commits_day".