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".

Make options title/repository optional, add option -m/--max, major code cleanup, full PEP8 compliance

All changes:
- chart title and repository are now optional and must be set with two
  new options:
    -t, --title (default: hardcoded in script, depends on the chart)
    -r, --repo (default: '.', ie current directory)
- add option -m/--max: max different entries in chart: after this
  number, an entry is counted in "others" (only for charts "authors" and
  "files_type")
- if filename is "-", display SVG content on standard output
- import the new division operator and the print function
- replace %-formatting with .format()
- full PEP8 compliance
- major code cleanup: add comments, dynamically build the names for week
  days and months, use a dynamic name for functions building a chart.

Add PNG support